Chapter 1 | My Little Sister Lost Her Memories Just Before Starting High School

“—Why am I the only one who’s different from everyone else? Am I not really Onii-chan’s family?”

The memory felt distant, like a faint echo from long ago.

Back in kindergarten, Yuuri had sobbed those words, teased for the silver hair that set her apart. It was a trait she was born with, a quirk of her constitution, their parents had explained when Mahito asked out of childish curiosity.

—That’s why, Mahito. If Yuuri’s ever troubled because of her hair color, help her out, okay?

As her big brother, it was his duty to protect her. So, with a gentle smile, Mahito had reassured her.

“Don’t be silly, Yuuri. Hair color doesn’t make us any less family, you know?”

He carefully tied a black ribbon into her shimmering silver hair, his voice warm. “Besides, I love your hair, Yuuri. It’s beautiful.”

In the sunlight, her hair seemed almost transparent, like delicate lace curtains, with faint strands of red and gold woven in, creating that striking silver hue. It was a secret treasure, one only he truly appreciated.

Carrying her home on his back, her small body exhausted from crying, Yuuri clung to him tightly and whispered, “…Thank you, Onii-chan. …I love you.”

In that moment, he vowed to protect his little sister, no matter what.

◇

“…Feels like I had a nostalgic dream or something.”

Mahito stirred in the living room, roused by the sound of the front door opening. His smartphone slipped from his hand, still open to the mobile game he’d been grinding before dozing off. A quick glance at the clock showed it was just past five in the evening.

The western sky glowed orange, though daylight still lingered. Spring had stretched the days longer, a quiet sign of the season’s arrival. Through the window, the distant shouts of boys echoed, savoring the final days of spring break.

Only three days remained until the opening ceremony. For Yuuri, starting high school with her entrance ceremony, it was just two. Mahito’s lazy spring break was nearing its end, and a new chapter loomed ahead.

Yuuri stepped into the living room, and Mahito, striving to sound casual, called out, “W-Welcome back, Yuuri.”

His little sister, now on the cusp of high school, wore a pleated skirt, a green camisole, and a beige cardigan—an outfit that carried a hint of maturity. Even accounting for brotherly bias, she could easily grace the pages of a magazine. Her long silver eyelashes framed wide, expressive eyes, and her small face, no longer childish, radiated a refined beauty. She seemed far more poised than many of the second-year girls Mahito knew.

But her silver hair stole the spotlight, as always. Delicate and silken, it cascaded to her waist, distinct from Mahito’s own. Bathed in the evening light, it shimmered with a faint vermilion tint, adorned with black ribbons tied neatly on either side.

Yuuri carried a large paper bag—her new school uniform, no doubt. Her excellent grades had earned her a recommendation, and her uniform had been ready since February. But after trying it on, the fit wasn’t quite right, so she’d had it adjusted, leaving the pickup until this last moment.

“…………”

Her response was a fleeting, almost icy glance before she turned away without a word.

When they were younger, she’d trailed after him, her voice bright with “Onii-chan, Onii-chan!” But by middle school, that closeness had faded. Perhaps it was her rebellious phase. These days, even when Mahito tried to reach out, she barely acknowledged him. How long had it been since he’d heard her voice?

—Well, it’d be strange if she stayed glued to her big brother forever, right…?

The thought left a pang of loneliness, but Mahito accepted it as the natural drift of siblings. He didn’t press her to talk.

—Still, I heard she got teased about her hair in middle school too…

High school might bring the same challenges. Yuuri was likely more anxious about it than he was, but things weren’t like they were when they were kids. She could handle herself now, and Mahito stepping in would only feel like meddling.

Not long ago, he’d spotted her at the shopping mall by chance. Two tanned, bleach-blond delinquents—types more suited to the city center than their quiet neighborhood—were hitting on her. Heart racing, Mahito had moved to intervene, ready to protect his sister.

“…Huh? Could you not talk to me?”

Yuuri’s voice, sharp and cold enough to freeze, cut through the air. Her chilling glance sent the delinquents retreating, their bravado shriveled in an instant.

Even from a distance, Mahito couldn’t help but let out a startled “Hii!” at the sight of Yuuri’s glare.

He silently thanked the heavens he wasn’t the one on the receiving end of that icy stare. The delinquents, cowed, mumbled a shaky “S-Sorry” before scurrying off. Hidden behind a pillar, Mahito trembled, hoping those guys wouldn’t carry the scars of that encounter for life.

No way could he muster the courage to force a conversation with her after that.

—Yuuri would probably look amazing in her high school uniform, though.

The thought crossed his mind, but saying something like that to his teenage sister would likely earn him a look of pure disgust. So, Mahito kept his mouth shut and turned away.

“……?”

Normally, Yuuri wouldn’t even glance his way, but today, she seemed distracted, her gaze unfocused as if lost in thought.

—Oh, she’s going to trip…

The realization hit Mahito, and he instinctively called out, “Yuuri!”

“Wha—Ah!”

His warning came a fraction too late. Yuuri’s foot caught on the carpet, but she quickly steadied herself, avoiding a fall.

“……!”

Her sharp glare snapped toward him, piercing.

—Well, yeah, getting caught in an embarrassing moment would earn that kind of reaction…

Mahito quickly averted his eyes, pretending he hadn’t seen a thing. He reached for his phone, fumbling to ease the awkward tension, when something unexpected happened.

“—Um, Onii-chan.”

Yuuri’s voice broke the silence, catching him completely off guard.

Mahito nearly jumped, but he turned to her with the widest smile he could manage. “Y-Yeah? What’s up, Yuuri?”

It was likely the first real conversation they’d had in a year. Heart pounding, barely containing his nervousness, he waited as Yuuri fixed him with an intense stare.

“…………”

Her lips parted slightly, as if she meant to say something, but no words followed.

Unable to bear the silence, Mahito tried again. “W-What’s wrong?”

Yuuri took a small breath, then turned away with a quiet huff. “…It’s nothing.”

She poured herself a glass of water and headed back to her room.

—What was that about…?

Still, it was rare for Yuuri to speak to him at all. Maybe her rebellious phase was finally starting to soften.

—If it’s something important, I can ask her tomorrow or something.

With high school looming, Yuuri was probably feeling overwhelmed too. There’d be other chances to talk. Time was on their side.

For now, Mahito let himself feel a spark of joy that his sister had spoken to him for the first time in ages. That was enough.

He didn’t realize then that tomorrow wasn’t guaranteed to be like today.

◇

“—We have something important to tell you both.”

Late that night, Mahito and Yuuri were summoned to the living room. The clock showed it was past ten.

For Mahito, staying up late was no big deal, but Yuuri looked like she’d been roused from sleep. She rubbed her slightly puffy eyes, her expression guarded as she sat.

They took their places at the table, Mahito and Yuuri side by side, their father facing Mahito, their mother across from Yuuri. Though Yuuri still wouldn’t speak to him, she didn’t seem to loathe him entirely—she was willing to sit beside him, at least. The chairs, though, felt just a touch farther apart than necessary.

Their parents’ unusually formal demeanor set Mahito on edge, his gaze drifting nervously.

—This doesn’t feel like a “make up with Yuuri” kind of talk…

He didn’t think they were fighting, but to their parents, it might look that way. Still, the air felt too heavy for a simple sibling lecture.

To distract himself, Mahito glanced between his mother and Yuuri. The resemblance was striking. Yuuri’s icy demeanor contrasted with their mother’s gentle warmth, but their small faces, noses, lips, and eyes were near identical. The main differences were age and eye color—Yuuri’s carried a faint bluish tint.

Their mother’s silver hair, unlike Yuuri’s natural hue, was dyed, with faint black roots peeking through. When Yuuri was bullied as a child, their mother, incensed, had dyed her hair silver in defiance, a bold move to silence the taunts. She claimed it made her look younger, and indeed, her youthful appearance often led people to mistake her and Yuuri for sisters. On those days, her mood was noticeably brighter, a quiet pride in their resemblance.

Mahito, however, bore little likeness to either of them. His height and features had barely changed since middle school, leaving him stuck with a boyish look that invited teasing at school. He’d tried working out to shake off his insecurities, but the results were modest at best. Still hovering just over 160 cm, he wondered daily when his growth spurt would finally arrive.

Their father, by contrast, had the neat, polished look of an office worker—carefully styled hair, a lean face, and angular glasses that suited him well. Despite his desk-job appearance, his frame was surprisingly toned. Barely forty, he was younger than most of Mahito’s classmates’ fathers. Quiet and reserved, he wasn’t cold, just awkward, but Mahito knew his father’s care for the family ran deep.

Now, though, both parents wore expressions heavy with gravity, a sight Mahito had never witnessed before.

—Did something happen…?

The unfamiliar tension made him swallow hard. His father took a steadying breath before speaking.

“There’s something we’ve kept from you both. It’s important. Your mother and I decided we’d tell you once Yuuri started high school.”

Their mother nodded slightly, her face solemn. With Yuuri’s high school entrance ceremony just two days away, the timing made sense.

“I want to say first that your father and I truly care about both of you from the bottom of our hearts,” their mother added softly. “That’s why we decided to keep this a secret until you were older.”

The weight of their words told Mahito to brace himself. Glancing at Yuuri, he saw her tense, her nervousness mirroring his own.

—Maybe I should hold her hand or something…

But the days of such gestures were long gone. Doing so now would likely only earn him a look of disgust. Instead, he clenched his fist, his nails biting into his palm.

Their father, after a moment of watching them, spoke in a low, measured tone. “Your mother and I… we’re actually remarried.”

“Huh…? Remarried?” Mahito’s voice came out clumsy, caught off guard. Beside him, Yuuri’s large eyes blinked in surprise.

Their father nodded, his expression unwavering. “Back then, Mahito, you were a baby, and Yuuri was still in your mother’s womb.”

“…What?”

The words sank in slowly—remarried, Mahito a baby, Yuuri in their mother’s womb. A cold unease crept over him, his body stiffening as the implications began to form.

“Mahito, you’re my son from my previous marriage, and Yuuri is your mother’s child,” their father continued. “In other words…”

“You two aren’t blood-related siblings.”

The revelation hit like a physical blow, the ground seeming to vanish beneath Mahito’s feet.

—Wait, so Yuuri isn’t my real sister…?

She was still his sister, yet it felt as though a vital thread tying them together had snapped. The realization seemed to strike Yuuri even harder.

“No way…” Her voice trembled, barely audible.

With a clatter, Yuuri’s chair toppled as she shot to her feet.

“I don’t want that…”

“Yuuri, calm down. There’s more to—” their father tried, but she didn’t hear him.

“—!” Ignoring his plea, Yuuri fled the living room, her footsteps rapid and unsteady.

“Yuuri!” Mahito called, scrambling after her, a step behind.

—Is she heading outside?

She wasn’t going to her room—she was bolting for the front door. Slipping into sandals, Yuuri dashed into the night.

“Yuuri, wait!”

Mahito chased after her, barefoot, his heart pounding. As he reached out, Yuuri glanced back, her eyes brimming with large, glistening tears. Her lips parted as if to speak.

In that fleeting moment, he stretched for her hand.

A screeching sound tore through the air, and Yuuri’s body was flung upward.

—A moment too late, I realized she’d been hit.

“Yuuri!”

She lay motionless on the ground, thrown by the impact. Misfortune always strikes without warning, leaving no time to prepare.

That was the last moment Mahito shared with his “little sister.”

◇

“Man, getting hit by a car? That’s really clumsy, huh?”

The next morning, Yuuri, now in the hospital, let out a light laugh as she spoke. A bandage encircled her head, and a painful-looking gauze patch clung to her cheek, but those were her only visible injuries.

The accident had occurred at night in a residential area. The driver had slowed and slammed on the brakes, sparing Yuuri from worse harm. Miraculously, her injuries were minor. She was in a private room, an IV tube still in her arm, but she’d be moved to a shared room by the afternoon.

Even so, the sudden calamity had left the family reeling, clinging to prayers for her recovery. And that was her first comment of the morning.

Mahito let out a heavy sigh. —Well, she’s probably trying to act cheerful in her own way…

The accident was bad enough, but the news from the night before must have shaken her deeply. Yet, not wanting to burden the family, she put on this brave face—just as she always had.

Seeing Yuuri smile, even faintly, for the first time in so long brought Mahito a quiet sense of relief, despite the weight pressing on him. Still, as her brother, he felt compelled to chide her gently.

“Yuuri… ‘clumsy’ isn’t the word for it. Mother was half out of her mind with worry, you know?”

“Huh…?” Yuuri blinked, her expression blank, as if his words didn’t quite register.

When she’d been hit, their mother had sobbed and panicked, as though Yuuri had been lost forever. Their father, trying to soothe her, was visibly rattled himself. In the chaos, Mahito and the driver had been the ones to call for an ambulance and the police.

The driver, to his credit, had been remarkably composed. He’d guided the flustered Mahito to contact emergency services and stopped their mother from moving Yuuri, warning it could worsen her condition. He’d also handled the police and offered to cover hospital costs through insurance. Since Yuuri had darted into the street, Mahito almost felt guilty for the trouble caused.

Their father, now visibly relieved, softened his expression. “Anyway, I’m just glad you’re okay… No, it was our fault for springing that news on you without considering your feelings.”

Their mother, still trembling, pressed a handkerchief to her face, her voice breaking. “Really, I’m so glad… If something like that happened again, I’d be too scared to…”

“Mother, it’s okay now…” Their father wrapped an arm around her, though it seemed she’d need more time to steady herself.

But Yuuri, the center of it all, only looked confused. —Does she not realize how serious this was…?

Her gaze drifted awkwardly for a moment before she spoke again, her voice tinged with uncertainty. “Um…? What news? No, wait, more importantly…” She turned her eyes to Mahito. “Who… is this person? It’s not the driver, is it…?”

“““Huh…?”””

The words hung in the air, unmistakably directed at Mahito. Their father cleared his throat, a hint of reproach in his tone. “Yuuri, that kind of joke isn’t funny.”

But Yuuri tilted her head, her confusion deepening, as if she couldn’t grasp why she was being scolded. “Joke…? Oh, but you called her ‘Mother,’ didn’t you…?”

She tilted her head again, then clapped her hands lightly, a spark of realization in her eyes. With a faint blush, she said, “Then maybe you’re my boyfriend or something…”

“………………”

Their parents seemed to sense something was wrong, but the full weight of it hadn’t yet settled. Mahito, though, understood instantly.

—Yuuri wouldn’t say something like this as a joke.

Even with her distance, her cold glares, she’d never treat him like a stranger—not with her personality, not after the night before. Calling him her boyfriend, especially after months of silence, was unthinkable.

It could only mean one thing.

His voice caught in his throat as he stepped toward her bedside.

“W-Wait, um, uh…?” Yuuri looked up, her confusion growing.

Mahito’s hand found the grip-shaped switch on the wall by her bed. With a trembling force, he pressed it hard.

“Nurse, please come quick! This girl’s in critical condition!”

Mahito’s desperate call sent the hospital into a flurry of motion. His mind went blank, overwhelmed by the chaos.

He forgot why the accident had happened in the first place.

Forgot the reason Yuuri had fled into the street.

Forgot that the sister he’d known since birth wasn’t his blood sister, but his stepsister.

◇

“Psychogenic memory impairment…?”

[T/N: Psychogenic memory impairment is a memory dysfunction caused by psychological stress or trauma, rather than brain damage.]

Their father’s voice carried a note of disbelief as he echoed the doctor’s diagnosis. After a full day of exhaustive tests, the results were delivered that evening.

In the examination room, Mahito and their father sat across from the doctor, a gray-haired man with a calm demeanor. Their mother stayed with Yuuri—or rather, Yuuri was keeping her company. Mahito had joined their father to hear the findings.

On a glowing lightbox, images of brain cross-sections from an MRI were displayed. The doctor pointed to a specific area with a metal pointer, his expression impassive.

“There are traces of significant dopamine and norepinephrine release,” he explained. “It appears she experienced extreme stress just prior to the incident. This is likely the cause of the memory impairment.”

Turning to their father, he added, “Have you ever felt your mind go blank from nervousness?”

“Yes…” their father nodded.

“It’s a similar phenomenon, but far more severe. Honestly, this is the first case I’ve seen firsthand.”

The doctor’s words were clear, though hard to absorb. Mahito’s father pressed further. “And the accident… how does it factor in?”

“The accident was likely just a trigger. I don’t believe it’s directly responsible.”

Both Mahito and their father struggled to accept that the accident wasn’t the primary cause, leaving them momentarily speechless.

“Of course, she did hit her head,” the doctor continued, “so we’ll keep her under observation for about a week. And…” His gaze shifted to Mahito. “We’ll need to monitor the memory impairment further to be certain, but for now, she seems to recall her parents and herself.”

“So, what she forgot is…” Mahito’s voice trailed off.

“It appears to be only her brother.”

The words landed like a heavy stone in Mahito’s chest. She had forgotten only him.

It was true that things with Yuuri had been strained lately. She’d ignored him almost entirely, and Mahito had hesitated to bridge the gap.

—But to forget only me…?

Words failed him. Their father, maintaining his composure, asked, “Will her memory come back?”

“There’s no visible brain damage,” the doctor replied. “So, there’s a good chance it could return. However…” He paused, his tone softening but unsparing. “You should also prepare for the possibility that it won’t.”

The blunt honesty left Mahito reeling, his head bowed.

—Hey, Onii-chan—

Yuuri’s voice echoed in his mind, the words she’d started to say before the accident. Words he’d never hear completed.

Their father rested a hand on Mahito’s shoulder. “It’s not certain it won’t come back.”

“…Yeah,” Mahito managed, his voice hollow.

The doctor continued speaking, but the words drifted past Mahito, unheard.

◇

Yuuri remained in the hospital for a week of observation. During that time, high school began, and Mahito moved through his days like a shadow, attending classes but retaining nothing. His mind was elsewhere.

He chose not to visit her, a decision reached after discussions with their parents. Yuuri would likely feel uneasy with a brother she didn’t remember appearing at her bedside, and Mahito wasn’t sure how to face her either. They both needed time to grapple with their emotions.

But time marched on, indifferent to their turmoil. The week passed swiftly, and the day of Yuuri’s discharge arrived.

◇

“…What do I do?”

In the living room, Mahito clutched his head.

Today, Yuuri was coming home. Her memory still hadn’t returned.

Their mother had gone to pick her up from the hospital but was heading straight to work afterward. Their father was at work and wouldn’t be back until late. The only one home would be Mahito, free after classes.

That meant he’d be alone with Yuuri, facing her for the first time in a week.

He could kill time outside until their parents got home, but as her brother, he couldn’t just avoid her like that.

—I need to face her properly.

First, her forgetting him was because of the accident. It wasn’t Yuuri’s fault. So, he’d act normal.

—But… normal? What’s normal?

They hadn’t spoken properly in nearly a year—how was he supposed to talk to her? And to Yuuri, he was a complete stranger now.

Suddenly saying, “I’m your brother,” would probably be hard for her to accept. So, maybe he shouldn’t act too familiar.

—But what’s the right distance for not being too familiar?

The more he thought, the less he understood. Maybe he should’ve visited her during her hospital stay.

No, he’d agreed it was better for both of them to have time to sort out their feelings, hadn’t he? He’d wasted that precious time doing nothing, and that was his own fault. He knew that, but it didn’t tell him what to do.

As he writhed on the sofa, the doorbell rang. Yuuri was home.

—Whatever happens, happens!

There was no way to know what would come next. Mahito just had to steel himself.

With resolve, he stood at the front door to greet his sister. The doorknob turned with a click.

The first face to appear was their mother’s, holding Yuuri’s bags. Seeing Mahito, she paused for a moment before speaking.

“Mahito… You okay?”

Grateful for her giving him a moment to prepare, Mahito nodded. “I’m fine, Mother. I’m ready.”

“…Alright. Yuuri, come on in.”

His sister, whom he hadn’t seen in a week, looked a bit thinner. She probably hadn’t eaten much in the hospital. Understandable.

But Mahito took a small breath to steady himself.

—I thought carefully about what to say here.

So, he was able to say naturally, “Welcome back, Yuuri.”

The same words as the day of the accident, but ones he always said.

Yuuri blinked, slightly surprised, then replied with a hint of embarrassment, “I-I’m home… Um, Mahito-san.”

—Mahito-san—that way of addressing him drove home that Yuuri really didn’t remember him.

But he’d known that already.

Mahito took the bags from their mother. “Mother, I’ve got this.”

“…Okay. I’m counting on you.”

This past week had been tough on Mother too. She probably hadn’t had a moment’s rest. At the very least, he wanted to ease her burden by handling their own matters.

Seeing her off, Mother headed back to work.

Then, Yuuri spoke up, sounding troubled. “Um…”

“Yeah? What’s up?”

Though he responded, she hesitated for a few seconds, as if unsure.

Still, she gathered her resolve and asked, “Is it true that you’re my brother, but we’re not related by blood?”

“Huh…?”

It took a moment to process what she’d asked. Then, the reality slowly came back to him.

—That’s right… We’re not real siblings!

Their father’s attempt to prepare him must have included this too. But the shock of her forgetting him had completely pushed it out of his mind.

Mahito nodded with a strained smile. “Y-Yeah, that’s right. I only found out a week ago myself… Haha.”

Sometimes, all a person can do is laugh when things are hopeless.

And so, the brother who’d only just learned a week ago that his sister was his stepsister—with the added twist of her not remembering him—and the sister without any preparation or resolve, were reunited.