Prologue

A private hospital room on the top floor of a certain hospital. A delicate-looking girl lay in a bed surrounded by medical equipment, including a bedside monitor and an IV drip.

Her name was Saya Amamiya.

If she had been attending school, she would have entered high school the following year.

“…Yuya-niisan. Thank you for staying by my side all this time.”

“Hey, now. Don’t talk like it’s already over. You’ll jinx yourself.”

Sitting in a chair beside her bed, I playfully poked Saya’s cheek, then brushed the hair away from her eyes. Saya accepted the gesture with a ticklish little smile.

“I’m really grateful, you know? You came to visit me every single day without fail, and that made me really, really happy.”

“Of course I did. We’re family, aren’t we?”

Our parents had passed away a long time ago, and ever since then, it had been just the two of us. So when I learned that Saya was suffering from the same illness that had claimed our parents, I vowed to stay by her side until the very end.

That was what I genuinely wanted, and not once had I ever considered it a burden.

And that was why—

“I knew I shouldn’t have… I knew it was wrong, but I took advantage of your kindness because I didn’t have much time left. Even though I knew… I was being a burden to you.”

Saya’s words left me utterly astonished.

“What… What are you talking about? How could I possibly think of you as a burden?”

“You’re so kind. But I know, you know? You were supposed to graduate next year, but you dropped out of high school, didn’t you? You did that to make more time to visit me, right?”

“That’s…”

It was true that I had dropped out of school, but there were other circumstances I couldn’t tell Saya about. And yet… unable to explain them, I found myself at a loss for words.

“We’ve lived together all this time. I can tell even when you try to hide things. You’ve been looking miserable lately, and you’ve been sighing more often, too. I’ve noticed every single little thing, you know?”

“I-I might have looked that way lately, but it wasn’t because taking care of you was a burden or anything like that.”

“Then what’s the reason?”

“That’s…”

I knew that if I didn’t explain myself now, it would be practically the same as admitting that Saya’s suspicions were correct. Even knowing that, I couldn’t bring myself to speak. It was a secret I desperately wanted to keep to myself.

Besides, I was sure Saya would understand soon enough, even without an explanation. I figured I could clear up the misunderstanding later.

But—

Seeing me fall silent, Saya gave me a faint, feeble yet gentle smile.

“Niisan. I’m sorry… for causing you so much trouble. But… it’s all over now… So from now on… live for yourself… okay…?”

“…Saya? Hey, Saya?”

Something was wrong with Saya. Realizing that, I hurriedly turned my attention to the bedside monitor. I didn’t understand exactly what the numbers displayed there meant. But—

“…Alarms disabled?”

I frowned at the message displayed in one corner of the monitor.

“I asked the doctor… to turn them off… I wanted… our final goodbye… to be peaceful… just the two of us…”

“—!”

The moment I understood what she meant, I frantically reached for the alarm settings on the touchscreen. The instant I restored the settings, an ominous warning sounded throughout the room.

“…Live freely… for both of us… and… be happy… okay…?”

Those were Saya’s final words before she slipped into eternal sleep. She had died believing that she had been nothing but a burden to me, leaving behind a single wish that I could never fulfill.

A week later, after finishing Saya’s funeral and settling her affairs, I returned to the hospital room where she had been staying until just a few days earlier. This time, however, I wasn’t there to visit anyone. I was there to be hospitalized myself.

—That’s right. Hospitalized.

The illness that had afflicted our parents must have been hereditary. While visiting Saya, I had undergone a medical examination as a precaution, only to discover that I was suffering from the same symptoms as her.

It was an unknown disease with no established treatment and no hope of recovery. However, I was certain to live longer than Saya, whose condition had already deteriorated considerably. Once I learned all this, I kept the truth carefully hidden.

Because I didn’t want to make Saya, who would die before me, suffer any more than she already had.

But as a result, Saya had died under the mistaken impression that she had been a burden to me.

If only I could, I wanted to go back to that day and tell her the truth.

The reason I’d looked so miserable was that I was suffering from the same illness as her, and not once had I ever thought of her as a burden.

But that opportunity was lost forever. No matter how desperately I wished for it, I could never return to that moment.

So I thought about what I could still do. And then I remembered the modest wish Saya had expressed with her final breath. I wanted to grant that wish for her.

But—

“Live freely for both of us and be happy, huh…”

Lying on the bed, I gazed out at the same view Saya had once looked upon through the window.

The cityscape stretching beyond the enormous window resembled a miniature garden. And for me, it was the only world I could see anymore.

It was a world far too small for me to live freely and find happiness.

“…Besides, Saya isn’t even here anymore.”

Ever since our parents had died when we were little, Saya and I had lived together, just the two of us.

Thanks to the insurance money our parents had left behind, we never had to worry about living expenses, but life had been one hardship after another for two small children. To me, Saya had practically been my other half, and there was no way I could ever find happiness without her.

And so, I came to a decision.

I would build a lie upon lies.

I would spend whatever time I had left gathering all kinds of knowledge, studying each subject so thoroughly that it would be as though I had experienced everything myself. Then, someday, when I reunited with Saya in the afterlife, I would lie to her, telling her that I had enjoyed all sorts of wonderful experiences and lived a happy life.

Looking back, it was a poorly thought-out plan, riddled with flaws that anyone could have pointed out. But at the time, I couldn’t see any other choice—

And so, throughout the final year I spent in that hospital room, I devoted myself entirely to gathering knowledge.