For ten stolen minutes, the only sounds were her ragged breathing and the frantic thumping of his own heart.
"Clock's ticking, big bro," she whispered without opening her eyes. "Better enjoy the cuddles while they last." Her hand squeezed his, a sticky, complicated promise.
"Well… thanks," he murmured, the words feeling utterly inadequate.
He pulled out his phone, the screen's glow a harsh intrusion in the dark room, and transferred the remaining payment.
Then, reluctantly, he got up from the bed. "Gonna go back now. Sweet dream, Yuna." He leaned in, pressing a soft, fleeting kiss to her forehead before pulling away.
Yuna let the kiss linger for a heartbeat before her hand shot out, her grip like a vise on his wrist. Her eyes snapped open, bleary but sharp. "Hold up."
She wiped her forehead with the back of her hand, then deliberately smeared the mixture of sweat and his saliva onto his shirt.
"There," she declared, her voice a triumphant hiss. "Now you smell like sin too."
As he climbed out the window, a final, chilling taunt followed him into the night.
"Sweet dreams, onii-chan! Dream of my pajamas… and jail time."
The last thing he heard before escaping back to the relative safety of his own room was a disgusted mutter: "Ugh… sticky…" followed by the unmistakable thump of her cum-soaked bunny pajamas hitting the floor.
The ten minutes of stolen warmth in Yuna's bed felt like a fever dream.
The moment Makoto slipped back through his own window, the cold reality of what he'd just done crashed down on him.
He stood in the middle of his room, the scent of Yuna, of their shared sin, clinging to his shirt like a shroud. He felt hollowed out, filled with nothing but shame and a lingering, phantom pleasure that made his skin crawl.
He didn't sleep. He couldn't.
He sat on the edge of his bed, watching the moon crawl across the sky, listening to the quiet, domestic sounds of the house: the hum of the refrigerator, the gentle snores of their mother from the next room.
Each sound was a dagger in his conscience, a reminder of the sacred space he had just so thoroughly defiled.
The next morning, he was a ghost. He moved through the kitchen on autopilot, making coffee, his hands trembling slightly.
When Yuna finally emerged from her room, looking deceptively normal in an oversized sweater and shorts, their eyes met across the breakfast table.
For a split second, something flickered in her gaze, a flash of the previous night's raw vulnerability, before her usual mask of bratty indifference snapped back into place.
"Morning, perv," she mumbled, grabbing the milk from the fridge.
She didn't mention the new sheets on her bed, or the faint, purplish bruise blooming on her cheek where he slapped her.
They ate in a tense, charged silence, the unspoken events of the night hanging between them like a thick, suffocating fog.
Later that week, he was walking home from his part-time job, his mind a swamp of guilt and exhaustion, when he saw her.
She was with a group of friends, laughing at something one of them had said.
She looked normal. A normal college student, bright and vibrant, her gray twin-tails bouncing as she laughed.
For a moment, he saw her not as his co-conspirator in depravity, but as the girl she was supposed to be, his little stepsister, the girl he was supposed to protect.
Then, a boy from the group, a handsome one with an easy smile, leaned in and said something that made her blush. He playfully bumped his shoulder against hers, and she didn't pull away.
Something cold and ugly coiled in Makoto's stomach. It was a feeling he recognized with a sickening lurch: jealousy. Raw, possessive, and utterly irrational.
He wasn't just her customer. He wasn't just her brother. He was something else. Something that couldn't bear the thought of another boy making her blush like that.
The realization was a punch to the gut. This wasn't just a game or a transaction anymore.
He had crossed a line, and the feelings that were now blooming in the dark, fertile soil of their shared sin were real. Terrifyingly real.
He watched them walk away, a knot of self-loathing tightening in his chest.
"What future did she have if she continued down this path?" A path he had not only encouraged, but actively funded and participated in. "What if she met some other guy, someone who wasn't just playing a role, but who was genuinely dangerous?"
The thought was a bucket of ice water over his head.
He had to stop this. Not just for his own sanity, but for hers.
He had to try and put the demon back in the box, to rewind time and go back to being just a normal, boring stepbrother and stepsister.
He knew it was probably impossible. He knew she would fight him, blackmail him, tease him until his resolve crumbled.
But he had to try. For her sake. And, if he was being honest with himself, for his own.
Because the thought of her being with someone else, of anyone else touching her the way he had, was a pain far worse than any guilt.
"I was falling for her! Not Ganyu, not Klee, but Yuna." He thought, and realized that was the most dangerous game of all.