The stale air of the detention room hung heavy, thick with boredom and the faint scent of floor cleaner. Outside, the afternoon sun dripped gold onto the U.A. High campus, a stark contrast to the drab interior where Y/N and Kirishima were serving their sentence.
It had all started with a playful shove during hero training. Y/N, ever the energetic one, had been sparring with Kirishima, their friendly rivalry always pushing them both harder. A well-aimed feint had thrown Kirishima off balance, and he'd retaliated with a tickle attack. Laughter had erupted, distracting them both, and a stern glare from Aizawa-sensei had sealed their fate: detention.
Now, slumped in their chairs, the silence was almost unbearable. Kirishima, bless his earnest heart, had tried to engage Y/N in a conversation about the latest hero rankings, but she was too restless, her energy buzzing beneath her skin. He'd eventually succumbed to the lull, his head lolling onto his chest, soft snores escaping his lips.
Y/N watched him, a mischievous glint in her eyes. Kirishima looked utterly peaceful, his usually spiky crimson hair softened in sleep. An idea sparked in her mind, a playful urge to tease him, to shake him out of his slumber.
It started innocently enough. She reached out and gently brushed a stray strand of hair from his forehead. Then, emboldened, she lightly rubbed his thigh, the denim of his uniform rough beneath her fingertips. Kirishima stirred slightly, a soft groan rumbling in his chest, but he didn't wake.
Y/N's heart pounded a little faster. She trailed her hand upwards, across his toned abdomen, feeling the firm muscles beneath. Reaching his chest, she traced the outline of his pectoral muscles, a playful smile dancing on her lips. Kirishima shifted again, his breathing becoming heavier.
The temptation was too much. With a deep breath, Y/N stood and straddled his lap, her jeans-clad thighs pressing against his. She began to grind against him, a slow, deliberate movement. A strangled groan escaped Kirishima's throat, even in his sleep.
A blush crept up Y/N's neck. This was definitely crossing a line, but she couldn't seem to stop herself. The feeling of his body beneath hers, the way he reacted to her touch, was intoxicating.
She continued her teasing assault, her movements growing bolder. Kirishima's hands clenched into fists, his brow furrowed in concentration, the picture of a man fighting his way out of a dream.
Finally, with a gasp, his eyes snapped open. He stared up at Y/N, his expression a mixture of confusion, surprise, and… arousal. He was hard, undeniably and visibly so, pressed against her.
"Y/N?" he croaked, his voice thick with sleep and desire.
Y/N froze, her playful mood evaporating, replaced by a wave of embarrassment and a surge of heat between her legs. "Kiri… I…"
She tried to stand, but Kirishima's hands shot out, gripping her hips, holding her in place. "Wait," he said, his voice rough. "Don't move."
His eyes burned into hers, and she knew there was no turning back. The air crackled with unspoken tension, the playful atmosphere replaced by raw, unadulterated desire.
"What… what were you doing?" he asked, his voice a low rumble.
Y/N avoided his gaze, her cheeks burning. "I… I was just… teasing you."
"Teasing?" Kirishima repeated, a slow smile spreading across his face. "Is that what you call it?"
He tightened his grip on her hips, pulling her closer. "Well, I have to say, it was very effective."
He leaned forward, his lips brushing against her ear. "So effective, in fact, that I think I deserve a little… repayment."
He gently nipped at her earlobe, and Y/N shivered, her body responding instantly. "Kiri…" she whispered, her voice trembling.
He didn't give her a chance to say anything more. He tilted her head back and captured her lips in a searing kiss. It was nothing like their playful interactions before. This was intense, passionate, and demanding.
Y/N met his kiss with equal fervor, her hands tangling in his hair. She felt a surge of adrenaline, a thrilling mix of fear and excitement. This was so wrong, so reckless, but she couldn't bring herself to care.
Kirishima deepened the kiss, his tongue tracing the outline of her lips before plunging inside. Y/N moaned softly, her body arching against his. He pulled her even closer, grinding his hips against hers, letting her feel the full extent of his arousal.
The detention room seemed to shrink around them, the world outside fading away. There was only the two of them, lost in a whirlwind of desire.
He broke the kiss, gasping for air. "Y/N," he whispered, his voice ragged. "I want you. Badly."
Y/N looked into his eyes, saw the sincerity and the longing reflected there, and knew she wanted him too. "Kiri," she breathed, "I…"
Before she could finish her sentence, the door to the detention room swung open. Aizawa-sensei stood in the doorway, his eyes narrowed, his expression unreadable.
"Kirishima, Y/N," he said, his voice flat. "Detention is over. Get out."
Y/N and Kirishima scrambled apart, their faces flushed, their clothes disheveled. The awkwardness was palpable, the unspoken desires hanging heavy in the air.
They mumbled their apologies and hurried out of the room, avoiding each other's gaze.
The walk back to the dorms was silent and strained. Once they reached Y/N's door, Kirishima paused, turning to face her.
"Y/N," he said, his voice hesitant. "About what happened in there…"
Y/N bit her lip, her heart pounding. "We… we probably shouldn't talk about it," she said softly.
Kirishima stepped closer, his eyes searching hers. "I can't just pretend it didn't happen," he said. "I've wanted you for a long time, Y/N. And I think you want me too."
Y/N couldn't deny it. She nodded slowly, her cheeks burning.
Kirishima smiled, a genuine, heartfelt smile that made her knees weak. "Then maybe," he said, "after all this hero stuff, we can explore this... more privately?"
Y/N laughed, a nervous but relieved sound. "Maybe," she said. "Maybe we can."
And with that, she stepped into her room, leaving Kirishima standing in the hallway, a hopeful smile on his face. The detention may have been over, but their story was just beginning. The crimson blush of confession had painted a new landscape on their relationship, and they were both eager to explore it.