If the main character of a story is a male high school student, their seat must always be by the window. By the window, around second from the back.
A certain student of the private Yumoto Academy"s high school division, Kirishima Chizuru just happened to sit by the window, two seats from the back. He had only just entered his second year in the high school division, and at present, the light tree-filtered sunlight of spring was pouring into the cla.s.sroom, and Chizuru was taking an afternoon nap. To the surprise of none, this could also be called part of the standard formula.
The one who sat beside him, Beppu Ageha seemed resigned at the sight of Chizuru"s sleeping face. Cla.s.s was still in session. But be that as it may, she and Chizuru had been together through the middle school division, enough time for anyone to grow accustomed to his conduct.
The math teacher who spoke on and on of imaginary numbers caught the sleeping boy in his sights, and slowly made his way to his seat.
Sensing a presence, Chizuru slowly raised his head.

"Kirishima… were you listening to what I was saying?"
"No. I wasn"t listening, what of it?"

His double-eyelids were still dangling in drowsiness. Once his long eyelashes leisurely lowered to cover his eyes, his body followed suite to cover his desk. The math teacher grabbed Chizuru"s shoulders and pulled him up.

"What"s with you, each and every time…"
"You say each and every time, but…"

Chizuru put up some vague back-talk.

"Teacher, you"ve only been appointed to this school this year, and it"s only the third cla.s.s we"ve had together. Just looking at the frequency, I doubt it"s that big of a problem."
"Quit splitting hairs. You were sleeping three out of three times, so that"s a hundred percent. Do you have any self-awareness that you"re in Yumoto Academy… the most prestigious prep school in the prefecture? What"s more, of the two hundred second year students, the special curriculum that only selects forty."
"Thank you for the courteous explanation teacher. Um, I am aware of it, but if the cla.s.s is on material I already understand, then I should set that time aside for sleep in order to restore my energy to study through the night at my own home: a decision beneficial for all sides, was there anything strange about it?"

A vein popped up on the forehead of the teacher in his thirties, his eyebrows twitching. For Chizuru"s fellow cla.s.smates who had grown accustomed to his conduct, they could predict what would come next.

"Everything was strange about it!"

Just as they thought, the teacher shouted out.

"People who don"t listen to lecture, no matter how good their grades may be, are destined to eventually fall! Your teacher has seen a number of those people!"
"In that case, it looks like I"ll be a valuable sample case you"ve never seen before in your teaching career."

As Chizuru said that without the slightest change of expression, the corners of the teacher"s mouth twitched.

"Meaning?"

"Despite being a, "person who doesn"t listen to lecture," my grades will never fall."

Unable to endure it any longer, the teacher pointed at the blackboard.

"What nonsense. If this lesson really is meaningless to you, then go solve the problem on the board! Right now! On the spot!"

Chizuru slowly leaned his body out to take a glance at the blackboard concealed behind the teacher"s form. The moment the board came into his sight, he opened his mouth. It was a s.p.a.ce of two and a half seconds.

"X equals negative one. Or three minus two i."

"… Hah?"

The teacher blankly opened his mouth. And as if to check if he had the question wrong, he turned to look back that board. On it was an overly complex geometric figure, accompanied by a word problem so tediously long one wouldn"t think it was formulated by anyone smart. The teacher had yet to write the answer.

"Huh, just now you… huh?"
"Didn"t you hear? X equals negative one. Or three minus two i."
"Eh? I mean, you… you… huh?"

Confused as he was, the teacher timidly put a hand to his mouth.
A few seconds later, with an awkward smile on his face, he returned to the podium. As if he couldn"t be at rest unless he was satisfied with this turn of events, he muttered to himself.

"What"s this, so you were listening to my lecture after all."

After that, Chizuru tried laying his head down a number of times, but every time, Ageha would wake him.
And with this and that, math cla.s.s came to an end, and as if leaping at his own desk, the boy took on a sleeping posture. As he did so, Ageha rose him with a poke.

"Ow… what is it Ageha? Cla.s.s is already over."
"No, it"s definitely over, but… Chizuru, you know, I know you"re smart, but why not listen to cla.s.s for now?"

Listening to Ageha"s sermon, Chizuru stretched out. His hair was on the longer side for a boy"s, hanging over to cover his right eye.

"There"s no helping it. I mean, I was reading books all the way to morning."
"The way you said that was cleverly cute."
"It"s not like I"m trying to act clever or anything…"

Chizuru and Ageha had been together through the middle school division, and they would often talk to one another. Unless it was absolutely necessary, Chizuru wouldn"t establish conversation with anyone. Therefore, it was mainly Ageha who started up these talks.

"Even so, I was surprised at that counterattack you launched on Itou-sensei. I knew you were a genius, but did you calculate that all in your head? I can"t think that was humanly possible."
"How terrible… I"m a proper human."
"Sorry, sorry… hey, more importantly!"

Even when he was the one who started into it, Ageha changed the conversation. The words that followed completely diverted anywhere that conversation had been flowing.

"That look on Itou-sensei"s face back there! He was really seething back there! I was sure Itou-sensei was a top, but it"s that strong front of his that would make his teary face all the better if he was on the receiving side!!"
"… Good night."

Knowing Ageha"s lecture on the romantic affairs between men would run for a long time, Chizuru decided to call it a night early.

Once sixth period was over, Chizuru and Ageha dropped by the library to kill some time. Chizuru wasn"t affiliated with any club, and Ageha"s newspaper club wasn"t active today. They spent around two hours in the library room, and by the time they left school it was already pa.s.sed six thirty.
From Yumoto Boulevard that had begun to show its evening bustle, they entered a sire road.

"Chizuru, you"re a genius, aren"t you."

Under the twilight-dyed sky, Ageha spoke up quite suddenly.

"What"s this, all of a sudden?"
"No, I was just thinking about it. Wondering what the best occupation to make use of your brains would be."
"… Hah?"

Chizuru abruptly turned to look at Ageha"s face. His eyes turned to the hairpin that adorned the left side of her head. It was made in the shape of a b.u.t.terfly, almost as if it was made specifically to stick the name to the person, or so Chizuru mused.

(TL: The word Ageha means swallow-tailed b.u.t.terfly.)

"I mean…"

Chizuru leisurely waved his hand in front of his face.

"Occupation? What"s that? You must be joking."
"I"m being serious here. With a head as good as yours… maybe a doctor?"
"Eh? No way… a job that deals with peoples" lives is way too heavy."
"Then a college professor."
"The responsibility of teaching others is heavy too."
"Politician."
"On top of heavy responsibility of the highest degree, I don"t want to be abused by the ma.s.s media."
"Eh~, but with a head of your level, even if you used it on a normal career, it would just feel like a waste!"
"… I never said I"d use it on a normal career either."
"Then what will you be?"
"Thinking about it."

Chizuru let out a long breath.

"Yeah, I"m aware I have a good head on my shoulders."
"R-right… when the person in question says it himself, I get this sort of indescribable irritation."
"That"s why I… a job where I don"t have to put in any effort at all, and can just use my mind for a high pay, that"s the sort of job I want."
"What"s that, your ideals are way too high. Does a job like that exist?"
"I haven"t found one yet, so I"m still searching."

Chizuru slowly scratched his head.

"You know anything?"
"There"s no way there"s a job as convenient as that."

As they carried out that unending conversation while they walked, the two of them had come before a vacant lot. The lot was overgrown with a thick screen of various forms of vegetation, and it seemed like quite a likely place to find strange insects.

"Even so, Ageha, that hairpin of yours sure is flashy."
"You think? I just wanted to try wearing an, "Ageha"."

As I thought, thought Chizuru.

It"s flashy, but did you get any warnings from the teachers?"
"Not at all, not at all. See, our school"s regulations are on the more lenient side."

That was definitely true. There were barely any restraints on attire. To an extent that Chizuru was able to wear a hoodie instead of a dress shirt under his blazer.
But isn"t yours a bit too showy, or so Chizuru was going to say, when Ageha suddenly raised an, "Ah," and pointed towards the vacant lot.

"Hey, Chizuru."
"What"s up, Ageha?"
"There"s someone lying flat over there."

True enough, in the thickets, they could see a man in reddish clothing lying face down.

"What do you think he"s doing?"
"… You think there could be someone else?"
"Oh my, Chizuru. You think it"s that sort of thing?"

From there, as if suddenly struck with inspiration,

"You think it"s another man? A love between men?"
"… Why do your thoughts always go in that direction?"
"Well I"m sorry for being rotten. But this is something of an incurable ailment, and…"

Unable to bear another long-winded speech, Chizuru entered the vacant lot.

"Wait, Chizuru. What do you plan to do if that really is what"s going on?"
"No, looks like he"s alone, and…"

Stopping in mid-sentence, Chizuru held his mouth shut. And he rushed forward.

"What"s wrong!?"

Ageha ran behind. And seeing that, they both swallowed their spit.

"… Looks like he"s dead."

Chizuru said it in a mutter. Sure enough, the man"s breath had ceased. The shirt that looked red was originally white, died with the blood flowing from the knife stuck in his back.

"This… can"t…"

Ageha was petrified. Chizuru took out his phone to contact the police as he inspected the body. And his eyes stopped at the end of its extended right hand.

At the end of the victim"s extended right hand, was a letter he presumable sc.r.a.ped into the ground.

What looked to be a simple letter X.

(TL: In the original text, the Dying message is a Y, therefore the t.i.tle of the chapter. To make the mystery solvable to an English audience, it has been changed to an X. Later references back to this case will still call it the Y Case.)