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The strangeness of Ishigami's letters in 容疑者Xの献身 ('The Devotion of Suspect X')

15 September 2017 (updated January 2018)

cover of Japanese version by Bunshun Bunko

In 容疑者Xの献身 yōgisha ekkusu no kenshin ('The Devotion of Suspect X') by Higashino Keigo (東野圭吾), the protagonist, Ishigami Tetsuya (石神哲哉), sends threatening letters to his neighbour Hanaoka Yasuko (花岡靖子) in which he deliberately adopts the persona of an obsessive stalker.

The letters are part of an elaborate scheme by Ishigami to distract police attention from Yasuko and her daughter after they unintentionally murder Yasuko's abusive ex-husband. The content of the four letters quoted in the book is unexpected and jolting and their style is odd and unnatural.

Here I want to look at the contribution of the language and style of the letters to Ishigami's chosen persona.


On this page:

A. Japanese original
B. Quirks of the letters
C. Translationese
D. Two levels of understanding
E. Significance of the linguistic register of the letters
F. Expressions of attitude/tone of voice


A. Japanese original

Four letters are found at Chapter 16, near the end of the book, after Ishigami finds that Yasuko has been dating an old admirer. Three are letters kept by Yasuko; one is found on Ishigami's computer. I quote the entire content, although only part of it is relevant to the issue of 'strangeness'. A fairly literal translation is shown below each segment.

The first letter:

最近、少し化粧が濃くなっているようだ。服も派手だ。そんなのは貴女らしくない。もっと質素な出で立ちのほうがよく似合う。それに帰りが遅いのも気になる。仕事が終わったら、すぐに帰りなさい。
Saikin, sukoshi keshō ga koku natte iru yō da. Fuku mo hade da. Sonna no wa anata rashiku nai. Motto shisso na idetachi no hō ga yoku niau. Sore ni kaeri ga osoi no mo ki ni naru. Shigoto ga owattara, sugu ni kaeri-nasai.

'Recently your makeup seems to be a bit heavy. Your clothes are also flashy. That's not like you. A simpler style suits you. Also, I'm concerned at your returning home late. When your work is finished return home immediately.'

The second letter:

何か悩みがあるんじゃないのか。もしそうなら、遠慮なく私に話してほしい。そのために毎晩電話をかけているんだ。私なら貴女にアドバイスできることはたくさんある。ほかの人間は信用できない。信用してはいけない。私のいうことだけを聞いていればいい。
Nanika nayami ga arun' ja nai no ka. Moshi sō nara, enryo naku watashi ni hanashite hoshii. Sono tame ni maiban denwa o kakete irun' da. Watashi nara anata ni adobaisu dekiru koto wa takusan aru. Hoka no ningen wa shin'yō dekinai. Shin'yō shite wa ikenai. Watashi no yū koto dake o kiite ireba ii.

'Do you have some kind of problem? If so, I want you to tell me without reserve. It's for that purpose that I'm ringing you every evening. As for me, there is a lot of advice that I can give you. You can't trust other people. You shouldn't trust them. You need to listen only to me.'

The third letter:

不吉な予感がする。貴女が私を裏切っているのではないか、というものだ。そんな事は絶対にないと信じているが、もしそうなら私は貴女を許さないだろう。なぜなら私だけが貴女の味方だからだ。貴女を守れるのは私しかない。
Fukitsu na yokan ga suru. Anata ga watashi o uragitte iru no de wa nai ka, to yū mono da. Sonna koto wa zettai ni nai to shinjite iru ga, moshi sō nara watashi wa anata o yurasanai darō. Naze nara watashi dake ga anata no mikata da kara da. Anata o mamoreru no wa watashi shika nai.

'I have an ominous premonition. It is that you may be betraying me. I believe that this is absolutely not true but if it were I would probably not forgive you. That's because only I am your ally. The only one who can protect you is me.'

In the fourth letter (on his computer) Ishigami writes:

貴女が頻繁に会っている男性の素性をつきとめた。写真を撮っていることから、そのことはおわかりいただけると思う。
貴女に訊きたい。この男性とはどういう仲なのか。
もし恋愛関係にあるというのなら、それはとんでもない裏切り行為である。
Anata ga hinpan ni atte iru dansei no sujō o tsukitometa. Shashin o totte iru koto kara, sono koto wa o-wakari itadakeru to omou.
Anata ni kikitai. Kono dansei to wa dō yū naka na no ka.
Moshi ren'ai kankei ni aru to yū no nara, sore wa tonde mo nai uragiri kōi de aru.


'I've determined the identity of the man you are meeting frequently. I think you can understand that from the fact that I have taken a photo.
I want to ask you. What is your relationship with this man?
If you say you are in a romantic relationship, that would be an outrageous act of betrayal.'
私が貴女のためにどんなことをしたと思っているのだ。
私は貴女に命じる権利がある。即刻、この男性と別れなさい。
さもなくば、私の怒りはこの男性に向かうことになる。
Watashi ga anata no tame ni donna koto o shita to omotte iru no da.
Watashi wa anata ni meijiru kenri ga aru. Sokkoku, kono dansei to wakare-nasai.
Samonakuba, watashi no ikari wa kono dansei ni mukau koto ni naru.


'Don't you know what I've done for you?
I have the right to give you orders. Break up with this man immediately.
If you don't, my anger will be directed at this man.'
この男性に富樫と同じ運命を辿らせることは、今の私には極めて容易である。その覚悟もあるし、方法も持っている。
繰り返すが、もしこの男性と男女の関係にあるのならば、そんな裏切りを私は許さない。必ず報復するだろう。
Kono dansei ni Togashi to onaji unmei o tadoraseru koto wa, ima no watashi ni wa kiwamete yōi de aru. Sono kakugo mo aru shi, hōhō mo motte iru.
Kurikaesu ga, moshi kono dansei to danjo no kankei ni aru no naraba, sonna uragiri o watashi wa yurusanai. Kanarazu hōfuku suru darō.


'For me as I am now, causing this man to meet the same fate as Togashi is extremely easy. I'm prepared and also have the means.
I repeat: if you are in a physical relationship with this man, I will not forgive that kind of betrayal. I am likely to definitely get revenge.'

B. Quirks of the letters

For a Japanese reader, the tone of the letters is chilling. Apart from being replete with menace and possessiveness, including a threat to kill Yasuko's suitor, the language itself is decidedly strange and hints at an abnormal, obsessive psychology. This can be seen in at least four characteristics.

1. Personal pronouns

Ishigami's consistent use of watashi 'I' to refer to himself and 貴女 anata 'you' (here written 'valued woman') to refer to Yasuko is sociolinguistically inappropriate in most styles of Japanese.

Unlike English, which has a closed set of personal pronouns ('I, you, he/she/it, we, you, they'), Japanese has historically used a variety of pronouns. In modern Japanese, 'I' can be expressed as ore, boku, watashi, あたし atashi, うち uchi, etc. 'You' can be expressed as あなた anata, あんた anta, kimi, お前 omae, おたく otaku, 貴様 kisama, etc. These range from the polite to the downright insulting.

The choice of pronoun depends on many factors. In speaking to his mates, a young male might refer to himself as 俺 ore, in speaking to teachers as 僕 boku, and in speaking formally (such as in a conference) as watashi. In addressing other people, he might say お前 omae to close mates, kimi to those younger than himself in a defined social relationship, and the contemptuous 貴様 kisama in a heated no-holds-barred confrontation (with a high likelihood of fisticuffs).

あなた anata has often been put forward in Japanese as the standard pronoun for 'you' in emulation of European languages. But あなた anata is anything but neutral in Japanese. In reality it tends to be preferred by women and is traditionally used by a wife to her husband. Spoken by a student to a teacher, for instance, あなた anata would sound familiar and disrespectful.

Moreover, Japanese often avoids personal pronouns. For second-person pronouns, it's common to substitute the person's name or title, or simply drop the pronoun altogether. Honorifics and other devices are often enough to clarify who is being addressed.

Against this background, Ishigami's use of watashi 'I' and 貴女 anata 'you' can only be called idiosyncratic. Using watashi in a letter is not in itself strange. More informal terms like 俺 ore or 僕 boku would be over-familiar. But 貴女 anata is questionable. As a man supposedly taking Yasuko under his wing, Ishigami might have addressed her as kimi 'you' or with respectful distance as 花岡さん Hanaoka-san. By choosing the supposedly 'neutral' second-person pronoun 貴女 anata, Ishigami conveys an impression of stiffness and formality.

This awkwardness is reinforced by the occurrence of watashi 'I' and 貴女 anata 'you' together, as in:

私は貴女を許さないだろう。
watashi wa anata o yurusanai darō.
'I probably won't forgive you.'

and

私が貴女のためにどんなことをした
Watashi ga anata no tame ni donna koto o shita
'What I did for you'

as well as

私は貴女に命じる権利がある。
watashi wa anata ni meijiru kenri ga aru.
'I have the right to give you orders.'

In Japanese, this doubling up of formal pronouns verges on the unnatural, much like the classic line that many non-Japanese speakers mistakenly believe is real Japanese:

    私はあなたを愛しています。
    watashi wa anata o aishite imasu
    'I love you'

2. Verb forms

In contrast to the curiously formal pronouns, the letters are almost entirely lacking in the social niceties expected in Japanese. Verb forms are familiar, plain, and direct, to the point of brusqueness.

This is most obvious in the use of plain forms of verbs in preference to polite forms, as in da rather than です desu, 似合う niau rather than 似合います niaimasu, つきとめた instead of つきとめました tsukitomemashita, する suru instead of します shimasu, and 思う omou instead of 思います omoimasu. In letters it would be more normal to use the polite form.

The letter also uses imperatives like 帰りなさい kaeri-nasai 'return home!' (first letter) and 別れなさい wakare-nasai 'break up!' (fourth letter). As with its pronouns, Japanese has a range of orders and requests available, from the super-polite to the rude. It is beyond the scope of this post to list all the ways that Yasuko could have been told to go straight home, but a few might be (ranged from painfully polite to bluntly imperative):

    すぐに帰って頂けませんでしょうか
    sugu ni kaette itadakemasen deshō ka
    すぐにお帰りになってください
    sugu ni o-kaeri ni natte kudasai
    すぐに帰ってください
    sugu ni kaette kudasai
    すぐに帰りな
    sugu ni kaeri-na
    すぐに帰りなさい
    sugu ni kaeri-nasai
    すぐに帰れ
    sugu ni kaere

Needless to say, Ishigami could also have chosen other ways of conveying his displeasure, for example by making suggestions or recommendations.

What Ishigami chose was -なさい -nasai, a familiar way of issuing orders that might be used among family or friends, especially to people younger than oneself. It is the kind of language used by a mother towards a child. It is not particularly impolite, although familiar in tone, and is less blunt than the straight imperative. Here it depicts a man speaking familiarly as though he has the right to order the woman around. In using this form, Ishigami comes across as overbearing and possessive.

3. The noncommital language of the threats using だろう darō

The third, most sinister aspect of the letters is the language of the threats. One occurs in the third letter:

もしそうなら私は貴女を許さないだろう。
moshi sō nara watashi wa anata o yurasanai darō.
'if that is so, I probably won't forgive you'

The second forms the culmination of the fourth letter:

もしこの男性と男女の関係にあるのならば、そんな裏切りを私は許さない。必ず報復するだろう。
Moshi kono dansei to danjo no kankei ni aru no naraba, sonna uragiri o watashi wa yurusanai. Kanarazu hōfuku suru darō.
'If you are in a physical relationship with this man, I will not forgive this kind of betrayal. I am likely to surely get revenge.'

The form だろう darō represents a subjective judgement of the degree of certainty. It indicates that, in the speaker's view, the statement that it is attached to is probably true but falls short of being 100% certain. It is often used out of politeness to avoid making categorical statements.

A speaker can, of course, use だろう darō to assess the certainty of his/her own future actions, or what his/her probable actions might be in a hypothetical situation. What is disturbing here, though, is that, even as he issues direct threats to Yasuko and her friend, Ishigami delivers these as a calm objective assessment of his own probable actions, as though he were an outside observer dispassionately weighing up the possibilities.

Curiously, just before the last threat using だろう darō, Ishigami uses a straightforward 私は許さない watashi wa yurusanai 'I will not forgive'. Even more curiously, the following sentence using だろう darō also uses the adverb 必ず kanarazu 'definitely, certainly, without fail'. Having clearly stated that he will not forgive Yasuko if she has 'betrayed' him and having clearly threatened to 'definitely' get revenge, it is both incongruous and unsettling to hedge his threat with だろう darō. The effect is ominous.

Phrasing his threat to kill Yasuko or her boyfriend in cold objective terms of what will 'probably' happen is arguably more frightening than a direct threat.

4. The logic

A fourth feature of the letters is the way that the text unfolds. Ishigami writes in short, uncomplicated sentences. At two places the results are decidedly awkward.

The first is the two sentences:

不吉な予感がする。貴女が私を裏切っているのではないか、というものだ。
Fukitsu na yokan ga suru. Anata ga watashi o uragitte iru no de wa nai ka, to yū mono da.
'I have an ominous premonition. It is that you may be betraying me.'

The split into two sentences is ungainly. They would sound more natural as a single sentence:

    貴女が私を裏切っているのではないかという不吉な予感がする。
    Anata ga watashi o uragitte iru no de wa nai ka to yū fukitsu na yokan ga suru.
    'I have an ominous premonition that you may be betraying me.'

The second example is なぜなら naze nara 'that's because' in the following sentences:

...もしそうなら私は貴女を許さないだろう。なぜなら私だけが貴女の味方だからだ。
...moshi sō nara watashi wa anata o yurasanai darō. Naze nara watashi dake ga anata no mikata da kara da.
'... if it were (true) I would probably not forgive you. That's because only I am your ally.'

Like the previous example, the second sentence purports to clarify the first. In this case the second sentence is not only awkward; it makes no sense in terms of ordinary logic. Being Yasuko's only ally would not normally be considered valid grounds for threatening her over involvement with another man. Ishigami's justification adds to the abnormal atmosphere of the letters.


C. Specific style: Translationese

Taken together, these linguistic features give the impression of a person who is socially inept, disengaged, psychologically detached, controlling, and possessive. But Ishigami's language is not some arbitrary collection of linguistic idiosyncrasies. As any Japanese reader will immediately recognise, much of it belongs to a distinctive linguistic register: the 'hybrid' language of students mechanically parsing and translating English into Japanese.

English-language education in Japan employs certain conventions (or crutches) to help students parse sentences. By translating according to fixed formulas or expressions, high-school and university students create an 'interlingual text' that helps them understand what they are reading. The resulting 'translationese' is a style of language removed from ordinary spoken and written Japanese.

Four of the linguistic features of Ishigami's letter writing style have their roots in these conventions.

Convention no. 1

One is the mechanical use of a fixed set of pronouns as equivalents of English personal pronouns. Despite the important subtleties of Japanese pronouns that we saw above, students use the following pronouns indiscriminately in deciphering English:

'I' = watashi
'you' = あなた anata
'he' = kare
'she' = 彼女 kanojo
'it' = それ sore
'we' = 私たち watashi-tachi
'you' (plural) = あなた方 anata-gata
'they' = 彼ら karera.
To translate a sentence like 'Where are you going?' into Japanese, students will use あなた anata for 'you', regardless of whether the question is addressed to a young child or to an adult of high status. This convention lies behind Ishigami's constant use of watashi and あなた anata in his letters.

Convention no. 2

A second convention is the omission of the normal features of polite speech. Interpersonal elements of politeness and distance tend to be treated as embellishments that can be added later. This encourages the use of plain forms for verbs and the straightforward Japanese form -なさい -nasai for imperatives. This is again designed to allow the student to grasp the meaning as easily as possible without worrying about social niceties.

The lack of social niceties, particularly the lack of politeness, is one of the most striking characteristics of Ishigami's letters.

Convention no. 3

A third convention is the use of the 'tentative' verb form だろう darō to translate English verbs in the so-called future tense (i.e., those using the modal auxiliary 'will').

The rationale for using だろう darō is impeccable. Japanese requires sensitivity to the truth value of a statement — whether it is certain, whether it has been heard second hand, or whether it is based on appearances. Since the future is inherently unknowable, predictions should not be stated as certainties; hence the use of だろう darō to indicate that the speaker does not regard a future event as 100% certain.

For example, students will translate an English sentence like 'It will rain tomorrow' as:

    明日雨が降るだろう
    ashita ame ga furu darō
    'It will probably rain tomorrow' / 'I think it will rain tomorrow'

This indicates that it is not 100% certain that rain will fall. In fact, this how Japanese weather forecasters are expected to speak. Where English says "Rain is expected tomorrow", Japanese forecasters habitually say 明日雨が降るでしょう ashita ame ga furu deshō, using the polite form of だろう darō.

In the same vein, students would translate 'He will go' as:

    彼は行くだろう
    kare wa iku darō
    'He will probably go' / 'I think he will go'

Unlike the categorical nature of the English, this gloss conveys the judgement that future actions are not certain.

While this convention works well on the whole, in one important case it falls down badly. That is when 'will' indicates volition rather than prediction. In English, a speaker who says 'I'll go' is not indicating future probability; he/she is expressing the intention to go. The listener can assume that, barring exceptional circumstances, the speaker has made a decision or even a commitment to go. In order to indicate volition or intention, the English sentence should actually be translated into Japanese as 私は行く watashi wa iku or, better still, 私が行く watashi ga iku. This uses the grammatical present tense to refer to a future action and indicates certainty.

When Japanese high-school students interpret 'I will go' as 私は行くだろう watashi wa iku darō 'I'll probably go', they are getting the message wrong. だろう darō denotes a high probability of the speaker going but doesn't indicate a decision or commitment to go. In a situation where it was absolutely essential that someone should go, a person who could only say 私は行くだろう watashi wa iku darō 'I will probably go' in Japanese would be either pressed to give a stronger assurance or passed over in favour of someone else.

Ishigami's use of だろう darō in his threats is strange in Japanese precisely because he is delivering a clear statement of intent for which だろう darō is inappropriate.

Convention no. 4

A fourth set of conventions or strategies relate to the connections between sentences.

According to one convention, where a sentence is used in English to explain a previous sentence but the connection is not made explicit, the construction というものだ to yū mono da 'it is the fact that' is typically used to clarify the connection. The two sentences from Ishigami's letters are a good example of this:

不吉な予感がする。貴女が私を裏切っているのではないか、というものだ。
Fukitsu na yokan ga suru. Anata ga watashi o uragitte iru no de wa nai ka, to yū mono da.
'I have an ominous premonition. It is that you may be betraying me.'

The putative English original for translated sentences like these would be:

    I have an ominous premonition. You may be betraying me.

Japanese adds というものだ to yū mono da 'it is the fact that' in order to show that second sentence explains the content of the premonition.

A second example is the habitual translation of English 'because' as なぜなら naze nara 'that's because' in order to preserve the original order of clauses. In Japanese, clauses indicating cause are normally sentence-initial, whereas in English they often follow the main clause. For instance, English says 'I didn't like it because it was sweet', whereas Japanese adopts the reverse order 甘かったから好きではなかった amakatta kara suki de wa nakatta 'because it was sweet, I didn't like it'. By using なぜなら naze nara 'if (you ask) why' and からだ kara da 'it's because', Japanese can be forced into the same order as English. Taking our example sentence:

    'I didn't like it because it was sweet'
    好きでなかった。なぜなら甘かったからだ
    suki de wa nakatta. naze nara amakatta kara da.
    'I didn't like it. If (you ask) why, it's because it was sweet.'

In both of the examples in Ishigami's letters, these devices are employed in a way that sounds superfluous or unnatural from the point of view of ordinary Japanese prose.


D. Two levels of understanding

Because the Japanese reader knows that Ishigami's style resembles a schoolboy's or schoolgirl's translation from English, the letters can be interpreted on two levels at once:

    1. The straightforward meaning in sociologically normal Japanese
    2. Its meaning in the hybrid language of student translations

The two levels coexist and echo off each other.

For example, coming across Ishigami's strange use of pronouns, the reader will notice that they are stiff and socially inappropriate. At the same time, he/she will recognise them as the kind of language that results when translating directly from English, accounting for the unnaturalness.

Similarly, the reader will immediately register blunt verb forms (especially imperatives) as stylistically and socially inappropriate, but will also associate this with translationese.

More importantly, while だろう darō makes Ishigami sound chillingly distant from his own actions, the reader is simultaneously aware that だろう darō is a direct translation of English 'will'. Ishigami's threat to get revenge is a translation of the putative English sentence 'I will definitely get revenge'. The knowledge that Ishigami is making a naked threat, as translated from the English, while at the same time setting himself apart from his own probable actions in terms of normal Japanese usage, helps account for the eerie effect of the letters.

Given the strangeness of the rest of the language, the Japanese reader will not be totally surprised at awkward constructions like というものだ to yū mono da and なぜなら...からだ naze nara ... kara da since these are both familiar mannerisms in translationese. The claim that Ishigami could not forgive Yasuko if she had relations with another man 'because he is her only ally' can be understood as a meaningless trait of that style. At the same time, in a single ingenious stroke it reveals the horrifying logic of the stalker. Yasuko is not allowed to live her life as she pleases as it would invalidate the stalker's sacrifice.

By adopting the peculiar register of student translationese, Ishigami greatly enhances the impact of his letters to Yasuko and their effectiveness in drawing responsibility for her ex-husband's death onto himself.


E. What does the linguistic register of the letters tell us?

But Ishigami's choice of student translationese has wider implications. While his chosen persona appears to be just another element in his elaborate plan to protect mother and daughter, few readers will fail to notice how closely the style fits Ishigami's character.

We know that Ishigami is a brilliant but intensely private person working for many years as a high-school teacher, a job that wastes his true potential. We know that he spends much of his spare time alone bringing his formidable logical intellect to bear on his true passion: abstruse mathematical problems. We know that he relates poorly to people, including the school administration, the students in his class — and Yasuko, the object of his unrequited, probably unrequitable passion. We note his increasing obsession with Yasuko's other suitor.

Ishigami's choice of student translationese is thus uncannily apt. When he uses the unformed language of high-school and university students to express himself he highlights his own arrested emotional and social development. The inept style of the letters reinforces the reader's impression of Ishigami as something of a misfit. The reader becomes increasingly aware that Ishigami resembles that well-known Japanese stereotype, the otaku.

Since the language of student translationese is associated with English, it is also a stereotype for how foreigners speak Japanese. By adopting this voice, Ishigami presents himself as an 'outsider', one who is beyond the ordinary conventions of Japanese society.

The letters also highlight Ishigami's obsession with logic. In Japan, there is a traditional view of English as 'logical' and 'analytical' compared with the supposed subtlety and sensitivity to human emotion of their own language. The clunkiness of translationese has encouraged that view. Direct translations from English, particularly the compulsory and obtrusive use of pronouns and logical conjunctions, give the impression that English is mechanical, rigid, and lacking in social subtlety and warmth. Such translations sound to Japanese ears like the writings of a robot. This is indeed how Ishigami's letters sound — logical, cold, and lacking in the normal social expression of human emotion. Ishigami would, as Yukawa observed, do whatever was logically necessary to carry out his plan.

The letters mark an important turning point as the book moves towards its close. In The Devotion of Suspect X there is no omniscient author; the narration takes place from the perspective of different characters as they appear, allowing Higashino to manipulate the reader's knowledge through selective presentation. The letters form part of the author's escalating revelations, providing a deeper glimpse into Ishigami's mentality, bringing the realisation that the persona is not entirely a mask, propelling the story to its shocking dénouement...


E. Beyond translationese: Expressions indicating attitude/tone of voice

While 'translationese' sets the basic tone for the language of the letters, they also contain other modal elements that express a tone of voice or attitude and serve to form a brief narrative within the letters.

First Letter

The first letter, which admonishes Yasuko for her clothing and makeup and for coming home late, uses only declarative (indicative) forms and an imperative with なさい -nasai. The overall tone is calm and equable while clearly controlling.

Second Letter

In the second letter, Ishigami adopts a tone of sympathetic understanding, pretended or otherwise, for what might lie behind Yasuko's behaviour. The letter uses more colloquial language, as though speaking to her directly.

何か悩みがあるんじゃないのか。
nanika nayami ga arun' ja nai no ka.
'Do you (perhaps) have some kind of problem?'

あるんじゃないのか arun' ja nai no ka is close to the spoken language, especially the segment んじゃない n' ja nai, which is the colloquial pronunciation of the more formal のではない no de wa nai. This gives Ishigami's attitude a friendlier tone.

Ishigami explains that his frequent phone calls are motivated by a desire to help:

そのために毎晩電話をかけているんだ。
sono tame ni maiban denwa o kakete irun' da.
'That's why I'm ringing every evening'
いるんだ iru n' da has an explanatory function in Japanese, here captured with the rough gloss 'That's why I'm...'. It's somewhat assertive and self-justifying and has a colloquial tone, but it's not unfriendly. The corresponding full form, いるのだ iru no da, would sound over-formal and unnatural.

Ishigami then attempts to convince Yasuko not to rely on others:

ほかの人間は信用できない。信用してはいけない。私のいうことだけを聞いていればいい。
hoka no ningen wa shin'yō dekinai. Shin'yō shite wa ikenai. Watashi no yū koto dake o kiite ireba ii.
You can't trust other people. You shouldn't trust them. You need to listen only to me.

This string of sentence endings goes from 信用できない shin'yō dekinai 'cannot trust' (declarative) to 信用してはいけない shin'yō shite wa ikenai 'should/must not trust' (strong advice). He concludes with 私だけを聞いていればいい watashi dake o kiite ireba ii 'you should listen only to me', also an expression of strong advice.

Third Letter

The third letter contains expressions of emotion, including suspicion, anguish, and menace, ending with the threat not to forgive Yasuko if she has relations with another man.

貴女が私を裏切っているのではないか
Anata ga watashi o uragitte iru no de wa nai ka
'you might be betraying me'

This sentence expresses suspicion. のではないか no de wa nai ka is in written style, as compared with the more colloquial んじゃないか n' ja nai ka. Had Ishigami used the colloquial form, the letter would have sounded harsh and direct, as though he were talking at her. By using the written form, the message is equally direct but 'cooler' and more restrained.

Ishigami prefaces his threat not to forgive Yasuko with a conventional profession of disbelief in her 'guilt'.

そんな事は絶対にないと信じているが
sonna koto wa zettai ni nai to shinjite iru ga
I believe that such a thing is absolutely not true

This purports to give Yasuko the benefit of the doubt but is equally designed to cause a sense of guilt and panic. It conveys a sense of anguish but is also manipulative in intent.

Fourth letter

In the letter found on his computer, Ishigami has identified Yasuko's suitor and adopts a more aggressive tone, culminating in a threat to kill the suitor. The language uses more formal (written-style) constructions. The letter is accompanied by a photo or photos.

写真を撮っていることから、そのことはおわかりいただけると思う。
Shashin o totte iru koto kara, sono koto wa o-wakari itadakeru to omou.
'From the fact that I've taken photos, I'm sure you can understand this [that I've identified your suitor].'

This uses the difficult-to-translate construction, おわかりいただける o-wakari itadakeru. おわかり o-wakari is a mild honorific expressing respect for the other person. いただける itadakeru is the potential form of いただく itadaku, which is the humble equivalent to もらう morau 'to receive'. Literally the meaning is something like 'receive your understanding', expressed respectfully. The whole can be roughly glossed as 'I think you can understand this for me'. The language is superficially polite but is less so in the context. In more ordinary English it carries the tone 'I'm sure you can understand'. The tone is cool and threatening.

Ishigami demands that Yasuko clarify her relationship with Kudo:

貴女に訊きたい。この男性とはどういう仲なのか。
Anata ni kikitai. Kono dansei to wa dō yū naka na no ka.
'I want to ask you. What is your relationship with this man.'

The writer expresses forthrightly his desire to ask Yasuko about her relationship (貴女に訊きたい Anata ni kikitai 'I want to ask you'). Expressed so bluntly, this is the language of arguments in Japanese. The way the question ends, with なのか, is brusque and demanding. This tends to written rather than spoken style.

In another sentence, the Japanese asks aggressively:

私が貴女のためにどんなことをしたと思っているのだ
Watashi ga anata no tame ni donna koto o shita ka to omotte iru no da
'What do you think I've done for you?' — meaning 'Don't you know what I've done for you!'

The question ...と思っているのだ ...to omotte iru no da 'what do you think that...!' (or more realistically 'don't you know?') is similarly straightforward and aggressive. Although abrupt, it is a written rather than spoken form (のだ no da rather than んだ nda).

Finally Ishigami reveals that:

さもなくば、私の怒りはこの男性に向かうことになる。
Samonakuba, watashi no ikari wa kono dansei ni mukau koto ni naru.
'If not, my anger will be directed at this man'

Ishigami avoids directly stating his own intentions. Instead, he frames his predicted actions as a natural result of circumstances (ことになる koto ni naru 'will come about that'). This again reproduces the atmosphere of 'hedged threats' found at other letters.

In summary, while translationese is used throughout the letters and lends them a robotic tone, there are clear and important differences in emotional tenor in the four letters. Ishigami moves from a neutral but controlling tone in the first letter, to a conciliatory tone using spoken forms in the second, to the expression of strong emotion (mixing anguish and menace) in the third, to an angry and threatening tone with less friendly, more formal written language in the fourth.