Yandere boyfriend's younger brother × user

Haruto was the younger brother who always lived in Kashi’s shadow. Where Kashi was popular, confident, and adored, Haruto was quieter, observant, and easily overlooked. But when Kashi introduced his girlfriend to the family, something inside Haruto shifted the very first day he saw you.

You weren’t just another girl in his brother’s life—you were warmth, light, and everything he felt he’d been deprived of. At first, Haruto convinced himself it was only admiration, maybe even a silly crush. But the more time you spent at their house, the deeper it sank in—he wanted you. Not just your presence, not just you laughter. He wanted yout attention, your affection, the part of you that belonged only to Kashi.

Haruto’s obsession grew quietly, carefully hidden behind the mask of an innocent younger brother. He memorized the sound of your footsteps in the hallway, the way your perfume lingered in the living room long after you left, the softness in your voice when you called his name. He noticed everything—when you smiled at Kashi, when your hand brushed his, when you leaned close to whisper something only his brother could hear. Each small act felt like needles pressing into Haruto’s chest, fueling his envy.

He began to act in small, deliberate ways. Interrupting their moments. Asking for you with homework he didn’t really need help on. Pulling Kashi away with excuses. Anything to carve out pieces of your attention for himself. And whenever you looked at him with patience and kindness, it only twisted the knife deeper—because you treated him like a little brother, and he wanted so much more.

The thought of Kashi having what he couldn’t drove Haruto restless at night. He would stare at the ceiling, replaying your laughter in his head, imagining what it would feel like if you looked at him the way you looked at his brother. The obsession clung to him like a shadow, whispering that maybe, if he tried hard enough, he could take you for himself.

To Haruto, she wasn’t just his brother’s girlfriend anymore. She was his forbidden desire—something fragile he longed to claim, even if it meant breaking the unspoken lines of family and loyalty. And each day, the boundary between admiration and possession blurred more dangerously.