The door to Yoichi's room clicked shut behind him. In the aftermath of his crushing defeat, the Nakano sisters gathered in the spacious living room, the energy still crackling with tension from dinner's unexpected conclusion.
Nino attacked the already spotless kitchen counter with a cloth, her movements precise and aggressive. "Did you see his face when he realized we tricked him? Priceless." She scrubbed harder at an invisible stain. "What an arrogant jerk."
Yotsuba bounced around the coffee table, arranging and rearranging coasters with boundless energy. "I thought it was really cool how focused he looked! Like a detective in those crime shows!" She placed a coaster down with a flourish. "And he's super tall too!"
"He's not that special," Nino muttered, tossing her hair. "He's just some random guy Dad dumped on us."
Itsuki sat at the dining table, her planner open before her, color-coded tabs marking various sections. "Whether we like it or not, Father has made his expectations clear. We must maintain a civil and productive living environment." She clicked her pen three times, a nervous habit. "Our academic improvement is non-negotiable."
"Civil?" Nino slammed the cloth down. "He walks in here like he owns the place and you want civil? He practically insulted my cooking!"
"But he said 'compliments to the chef,'" Yotsuba pointed out, her green ribbon bobbing as she tilted her head in confusion. "That's a good thing from a brother, right?"
Nino ignored her. "And he had the nerve to fall for our trick and then act like it was beneath him."
Ichika sprawled across the massive sectional sofa, scrolling through her phone, a satisfied smile playing on her lips. "He's not our brother, Yotsuba. He's our step-brother. There's a difference."
"What difference does that make?" Nino shot back.
"All the difference in the world," Ichika replied, not looking up from her screen.
Miku sat at the far end of the sofa, headphones around her neck, fingers absently tracing the cushion pattern beneath her. "He's observant," she said quietly, her voice cutting through the bickering. "He noticed your slip-up instantly, Nino." Her eyes narrowed. "He's dangerous."
The room fell silent as they considered her words.
"Dangerous how?" Yotsuba asked, suddenly still.
"He sees things most people miss," Miku said. "Like a general surveying a battlefield."
Yotsuba's eyes widened. "Whoa, that sounds so cool!"
"It's not cool," Nino snapped. "It's creepy."
"So what are you gonna do with him, Ichika?" Yotsuba asked, bouncing back from concern to excitement in record time. "Since you won the bet? Can we make him do all the chores? Or maybe he could run laps with me! Or help with homework!"
Ichika finally looked up from her phone, a slow, almost predatory smile spreading across her face. "That," she paused dramatically, "is a secret. You'll all find out tomorrow."
"Whatever," Nino huffed, turning back to her compulsive cleaning. "He's probably just sulking in his sterile little room, counting his allowance money. Dad's giving him five hundred thousand yen a month, can you believe that?"
A brief shadow passed over Ichika's features. "Or maybe he's looking at the picture on his nightstand."
The sisters fell silent.
"What picture?" Itsuki asked, pen poised mid-air.
"Him and a woman," Ichika said, her voice gentler than usual. "They were holding a trophy together. She had his eyes." She let the implication hang in the air. "I think it was his mom."
Yotsuba's smile dropped. "Oh."
Itsuki's pen stopped moving entirely. Miku's expression remained carefully neutral, but her fingers stilled on the cushion. Nino just scoffed, though her scrubbing slowed.
"So?" Nino finally said, cutting through the uncomfortable silence. "That doesn't give him the right to be a jerk."
"No," Ichika agreed, "it doesn't. But it might explain a few things."
More silence fell, heavier than before.
"Still," Yotsuba said, her voice taking on a dreamy quality, "I wonder what it'll be like having a real older brother around. I mean, I've always been curious!"
The others exchanged glances.
"He is NOT our brother," Nino insisted.
"But he kind of is," Yotsuba pressed, "since Dad—"
"Step-brother," Itsuki corrected. "And only in the technical sense."
"Whatever he is," Yotsuba continued, undeterred, "it could be fun! Like, imagine if he helped me with my training!" Her eyes glazed over slightly as the thought took hold...
Yotsuba saw herself sprinting around the school track, Yoichi jogging easily beside her in a matching green tracksuit. The setting sun cast long shadows as they completed their tenth lap.
"Yotsuba! Your hundred-meter dash time has improved by point two seconds! This is the power of youth!" Yoichi gave her a thumbs-up, his teeth gleaming unnaturally. "As a reward, we will do five hundred pushups together!"
"Yes, Onii-chan!" Yotsuba cried, tears of joy streaming down her face.
She snapped back to reality with a giggle. "That would be awesome!"
Itsuki adjusted her glasses, a slight flush on her cheeks. "If he has any academic ability, perhaps he could assist with our studies. Father did specify that was part of the arrangement."
In Itsuki's mind, they sat in a quiet, sunlit corner of the library. Yoichi pointed patiently to an equation in her textbook.
"You see, Itsuki, the key is to carry the derivative through both sides. Your approach is very diligent; you just need to apply it correctly." His voice was calm, reassuring.
The scene shifted to a quaint café where he slid a massive strawberry parfait toward her. "You earned this for acing your exam."
Itsuki blushed happily as she dug in.
Itsuki blinked rapidly, clearing her throat. "For educational purposes only, of course."
"You're all being ridiculous," Nino said, but her eyes betrayed a momentary flicker of something else.
The kitchen materialized in her mind. She was chopping vegetables, the knife slipping from her grasp. Yoichi moved with lightning speed, catching the blade before it hit her foot. The momentum brought him dangerously close, his arm braced against the counter, caging her in.
"Be more careful," he murmured, his voice low, his storm-gray eyes intense as they locked with hers. His face was inches away.
Her heart raced treacherously. He smelled nice—nothing like cup noodles at all. Wait, what was she thinking?
"G-get away from me, you stupid freeloader!" But her face burned red, and she made no move to push him away.
Nino shook her head violently, her cheeks flushing. "This is stupid. He's a temporary problem, nothing more."
Ichika watched her sisters with amusement. "You're all so transparent."
"What about you, Ichika?" Yotsuba asked. "What do you think having a brother will be like?"
Ichika's eyes grew distant for a moment.
She saw herself lounging on the couch, fixing Yoichi with her best teasing smirk. "Aww, is little Yoichi-kun lonely? Does he need his big sister to keep him company?" She tucked her feet underneath her, ready for him to stammer or blush like most boys did.
But he didn't react as expected. Instead, he walked over, leaned down, and whispered in her ear, his voice unexpectedly commanding. "Stop performing, Ichika. I see right through you."
Her playful smile vanished, replaced by a genuine, shocked blush. The tables had turned completely.
Ichika blinked, her usual composure momentarily shaken. She recovered with a careless laugh. "I think it'll be entertaining, at the very least."
All eyes turned to Miku, who had remained silent. She met their gazes steadily.
"What?" she asked.
"Your turn," Ichika prompted. "What do you think about our new housemate?"
Miku hesitated.
She stood before a display of samurai armor in a museum, speaking with unusual animation. "So Takeda Shingen's use of the 'Fūrin Kazan' banner was a masterful piece of psychological—"
"But tactically flawed," Yoichi interrupted. "It made his army predictable. Uesugi Kenshin exploited that at Kawanakajima by anticipating his movements based on that very philosophy."
Miku's eyes widened. He wasn't just listening politely. He was debating her. He understood.
"I think," Miku said carefully, "we should observe him further before drawing conclusions."
"Well, whatever he is," Ichika said, stretching languorously on the sofa, "he's mine for the weekend." A final, enigmatic smile played on her lips. "I hope he's a morning person. It's going to be a very, very long two days."
Nino crossed her arms. "What are you planning?"
"Nothing dangerous," Ichika assured her. "Just... educational."
"For him or for you?" Itsuki asked suspiciously.
Ichika just winked. "Both, hopefully."
"Just don't break him," Miku said, rising from the couch. "We still need him for tutoring."
"I make no promises," Ichika replied.
Yotsuba clapped her hands excitedly. "Can I help? Please? I could wake him up super early for morning exercises!"
"No, Yotsuba," Ichika said firmly. "This is my prize. You'll all have your turns with him eventually."
"That sounds wrong," Itsuki muttered.
"Only if you make it wrong," Ichika teased, causing Itsuki's face to turn bright red.
"Fine," Nino conceded, removing her apron. "Have your fun. But don't think this changes anything. He's still not part of this family."
"Of course not," Ichika agreed, but the look in her eyes suggested she had already begun considering possibilities none of them had anticipated. "He's just... a temporary problem we need to solve."
From the hallway upstairs came the faint sound of music—a soft, melancholy jazz tune floating down from Yoichi's room.
The five sisters paused, listening for a moment to the unexpected soundtrack of their new reality.
"Well," Ichika said, breaking the spell, "I should get some sleep. Tomorrow's going to be interesting."