Prologue

Every time Akashi Yurika moved, she commanded the attention of every student in the room.

It wasn’t just the boys; the girls were equally transfixed. It was as if they were all collectively under the spell of her sheer beauty.

This woman, who drew every eye without even trying, was a young high school teacher who had just taken over as a homeroom teacher this year.

She had long, lustrous black hair, wide and enchanting eyes, a perfectly straight bridge to her nose, and lips that looked soft and supple. "Perfect beauty" was the only phrase that truly fit her.

But Yurika’s charm didn’t stop at her facial features.

She possessed a bountiful chest that strained against her gray suit jacket from the inside, so much so that the faint pattern of her bra was almost visible through the fabric of her blouse. Her slender waist carved a sensual curve down to her hips, and the sheer volume of her rear against her tight skirt was enough to make the line of her underwear stand out.

To top it all off, there were her legs, encased in sheer black stockings. Stretching out from beneath her short skirt, those legs flaunted their long, beautiful lines with unapologetic grace. Combined with her shapely thighs, she radiated an overwhelming sense of sex appeal.

Buzz, buzz, buzz...

She was a teacher who carried herself with such an aura that one might wonder if her pheromones were actually visible to the naked eye. It wasn't hard to imagine how any adolescent boy would react to such a presence.

"Man, Akashi-sensei is something else today, too."

"Seriously, she's so cute."

"Those boobs... that ass... those thighs..."

"I'd give anything for some 'private lessons' from her..."

The boys whispered among themselves, their comments growing increasingly lewd. Of course, Yurika wasn't the type to let such tasteless gossip slide.

"You over there. Quiet down. I won't have you chatting during my lecture," she snapped.

Her icy words cut through the room instantly, and the boys bolted upright in their seats.

"S-sorry!"

"We'll be careful!"

Indeed, the rumors about Yurika went beyond her being a "perfect beauty." Because of her bewitching, glamorous looks and her dignified, sharp discipline, she was often referred to by nicknames like the 『Queen』 or the 『Succubus』.

"God, even that cool, 'Queen' voice of hers is amazing."

"Seriously, she's the best."

"It’s a mystery why she doesn’t have a boyfriend."

Despite their earlier reprimand, the boys continued to whisper, this time in hushed tones. To them, even getting scolded by her felt like a reward.

However, for all the talk about her being a perfect beauty, there was one mystery surrounding Yurika.

Despite her incredible looks and immense popularity, there wasn't even a shadow of a man in her life. It was almost as if she hated them entirely.

◆　◇　◆

Ding-dong-dang-dong—

As the chime echoed through the classroom, the students began to move all at once. Some headed for their club activities, others planned detours with friends, and some simply intended to go straight home to enjoy their private time.

"Whew... finally over."

Akashi Yuu began to pack his things.

He wasn't particularly handsome; if anything, he left a rather plain impression. However, for some reason, he often found himself being fussed over by older women or assertive girls. He had an innocent, "pure-virgin" aura about him—though it might be more accurate to say people just liked to tease him for it.

Naturally, Yuu was planning to head straight home.

"Yuu!"

Just as Yuu was about to stand up, his classmate, Chuujou Takami, called out to him. They had been classmates since middle school, and she was the type of girl who was always finding some reason to involve herself with him.

She had a healthy, fresh physique and glossy, semi-long hair. Her expression was lively, giving her a slightly aggressive, spirited image.

"Hey, Yuu! Are you even listening!?" Takami pressed, her voice insistent.

"Do you need something, Chuujou-san?" Yuu replied with a listless tone.

Takami’s voice went up an octave. "Hah?! Do I need a 'reason' to talk to you!?"

"Ah, no, that's not what I meant..."

"Exactly! When I call you, you answer immediately!"

Takami had a habit of picking on Yuu for one thing or another. Today was no different; she was pestering him over some trivial matter.

While it was true that Yuu was often "looked after" by older or assertive women, it wasn't that he triggered their maternal instincts—it was more like he triggered their S instincts. As if to prove the point, Takami leaned in close, her eyes glittering with intensity.

"Hey, your hair's getting long, isn't it? Why don't you cut it?"

"I feel like if I let you cut it, the results would be disastrous," Yuu deadpanned. "Like you'd get carried away and take my head off with it."

"What are you, a samurai in the Sengoku period!? Of course I wouldn't do that! Jeez, I'm just saying I could recommend a good hair salon to you."

"I don't really care about my hairstyle."

"Well, you should! You look like a total otaku like that. You'll never be popular with the girls."

"I don't really need to be popular..."

"No way! If you're going to be like that, it's embarrassing for me since we’ve known each other so long!"

Yuu had no idea why she would be the one feeling embarrassed, but she seemed hell-bent on dragging him to a salon regardless.

"You're free this Sunday, right? Then it's settled."

"Wait, no, nothing's settled—"

"Hold it right there!"

Just as he was about to be railroaded into a weekend plan, an unexpected intruder cut in.

"Akashi-kun! You still haven't submitted the career path survey that was due the other day, have you?"

"Eh? Onee-chan—I mean, Akashi-sensei!"

Yuu froze as he turned around.

The person who had spoken was Akashi Yurika—the homeroom teacher rumored to be a "perfect beauty" and a "queen."

And she was also Yuu’s older sister.

"We need to have a word. Come to the student counseling room later!"

With a prickly edge to her voice, Yurika gave him a stern look and marched out of the classroom, her dignified aura intact. Once she was sure the teacher was gone, Takami leaned in close to Yuu's face again.

"Man, Yuu, your sister is as strict as ever with you, isn't she? I wonder if that's it? Like, she's being extra hard on you so no one thinks she's playing favorites with her own family."

Yuu ignored his pushy classmate and let out a heavy sigh.

"Haaaah... Anyway, I'm going to the counseling room."

"Think about Sunday, okay!?"

Yuu left the classroom without giving a "yes" or "no." He knew that if he let things get more complicated, he’d only have to face a terrifying ordeal later.

◆　◇　◆

"Excuse me..."

Yuu spoke up before opening the door to the student counseling room.

"You’re here. Get in, quickly."

Yurika, still maintaining her prickly attitude, walked over to the door to usher him in. Her expression shifted into something slightly more mysterious as she reached behind her and turned the lock.

Click!

Once she confirmed the door was locked and no one could walk in, Yurika closed the distance with staggering speed.

"Yuu-kun!"

Squoosh!

Without a care for the fact that her soft, ample breasts were pressing right against him, she lunged forward and clung to him. Her expression and attitude were now the complete opposite of what they had been moments ago.

"Yuu-kun! Seriously, what was that?! You were totally flirting with that girl, weren't you?! Is a younger girl really that much better?! If you're feeling 'backed up,' your Onee-chan has told you a million times she'll make you feel good! Ohhh, Onee-chan is just so, so worried that some bad woman is going to snatch my Yuu-kun away!"

"Hey! You're too close! Get off me!"

The difference between the dignified, adult aura she projected in class and this borderline-broken, doting mess was like night and day. Yurika clung to him like a vine.

This was the truth about Yurika—

She was an incredibly "heavy" older sister with a level of brother-complex that bordered on the obsessive.

"Look, I was just making small talk with Chuujou-san. There’s nothing going on."

"Lies! That girl is definitely aiming for my Yuu-kun! She had the face of a female in heat! Yuu-kun, you have to be careful!"

Yurika blurted out something truly outrageous. If Takami had heard that, she probably would have fainted from shock.

"No, no, absolutely not! And what even is a 'face of a female in heat'?"

"Hmph! Yuu-kun, you are hereby forbidden from talking to girls!"

"That’s impossible! I can't live a normal school life like that! If you don't cut it out, I'm going to get mad."

"Waaaaah! S-sorry, Yuu-kun! Onee-chan was just worried about you..."

Squish! Squeeze!

"Fwaaa... Yuu-kuuuun...♡"

"Agh, you're hitting me! You're hitting me with... everything!"

As Yurika hugged him, pressing her chest and thighs against him, Yuu—who was at an age where his sexual impulses were at their peak—felt his self-control reaching its absolute limit.

After all, the two of them were step-siblings with no blood relation. Thanks to his parent's remarriage, he had suddenly gained an incredibly sexy and gorgeous older sister. On top of that, at home, Yurika would walk around in her underwear or just a bath towel, intentionally trying to seduce him. With a "dirty" sister like this, it was only natural that Yuu's patience was wearing thin.

Every day was a battle of stifled desires with no room for peace of mind.

"I said you're too close... Your boobs are hitting... Ah..."

"Fufu...♡ Yuu-kun...♡"

"I can see! I can totally see! Your cleavage!"

As far as Yurika was concerned, she was only exposing her defenseless side in front of Yuu on purpose. Yuu, unaware of her true intentions, couldn't help but steal glances while desperately trying to figure out where to look.

"Hah... hah... You're done with what you needed, right? I'm going home."

Yuu finally managed to pull away and moved toward the door.

"Ehhh?! Yuu-kun, let's go home together!"

"No! You're supposed to respect the boundaries between a teacher and a student at school. You're the one who decided that, Onee-chan... I mean, Sensei."

Naturally, a romance between a teacher and a student was strictly verboten. It was a rule as ironclad as the laws of the shogunate or the codes of the Shinsengumi. While he probably wouldn't be forced to commit seppuku, it would undoubtedly cause a massive scandal.

Plus, on the surface, the two of them were known as siblings. If their secret were ever exposed, they would be branded by society as a "lewd brother and sister" who had committed a taboo.

"Come on, just a little bit of flirting...♡"

"The answer is no."

Even though Yurika was the one who had insisted on maintaining their professional distance, she was now the one trying to sneak in some intimacy in the student counseling room. She truly was a troublesome sister.

"Yuu-kun, I'm sorry. I'm sorry for being so prickly in the classroom. It's just... if I don't discipline myself like that, I feel like I'll slip up and show my true self immediately!"

"I get it, so it's fine. I'm leaving now."

"Yuu-kun! See you later! Let's sleep together tonight!♡"

"We are not sleeping together!"

As Yuu stepped out of the student counseling room, he felt his heart racing.

Thump-thump, thump-thump, thump-thump—

(Ugh, she was so soft... And... she smelled so good... Damn it! I can't take this. What the hell am I supposed to do!)

The truth was, Yuu was a massive "Siscon" who absolutely adored his older sister.

When she had hugged him earlier, he had been using a will of steel to resist the urge to squeeze her back and shower her with kisses. His feelings for his sister were overflowing and becoming impossible to suppress.

(Haaa... what should I do? I'm at my limit. I feel like I might make a 'mistake' tonight. I don't want to go home... But I still want to go home and be with her! Haaa...)

Yes, this is a story—

A story of an older sister who uses every trick in the book to seduce her younger step-brother, and a younger brother who loves his sister dearly but desperately resists, thinking of it as a 『forbidden love』 and telling himself to 『wait until he can take responsibility』.

This is the record of a battle involving pride, convictions, and chastity—a battle of love, courage, and pure eroticism.