Warning: Sexual references and themes (not how you're thinking (particularly))
Based off one of my weak attempts at a BNHA comic on Instagram.

———

Shinsou yawned and stretched as the kettle slowly growled into life.
Blinking the sleep from his eyes he noticed for the first time he wasn't alone in the kitchen. Which at 4 am would be frightening for anyone.
He relaxed when he recognised the other presence as a cat, but then tensed up again when he realised it was not one of Aizawa's.

Its eyes were huge and green, and reminded him of Izuku's. They were so big they took up most of the poor cat's face, framed by ridiculously fluffy green fur.
He stared at the cat, and it yowled grumpily making him chuckle.

"Hey cutie, how did you get in here?" He murmured playfully, reaching out a hand to scratch behind the feline's ears. It glared at him, ducking its head, but he caught up and its resistance vanished at the first scratch, the cat melting onto the counter top and purring contentedly.

"Mmm," Shinsou smirked in satisfaction, "that nice?"
The cat huffed, stretching out more and growling a little when he went to pull away.
"Easy." He teased, brow furrowing a little, "no collar huh?"
The cat seemed to wake up a little at his question, shaking his hand away and getting up. It yowled at him expectantly, but when he blinked back, confused, it rolled its eyes.

Wait. It rolled its eyes.
"You can understand me?" He asked uncertainly.
The cat nodded, somehow its eyes were laughing.
"...you're a person?" Pressed Shinsou, wandering if he was hallucinating, wouldn't be the first time with his level of insomnia.
The cat managed to pout somehow, and nodded encouragingly.
"But none of the class can- Izuku! How the hell! This wasn't funny!"
The cat made a teasing yowl as if to say, actually it was hilarious.
"Okay," grumbled Shinsou, turning to prepare his coffee, "you got me. Turn back so you can laugh at me properly."
When there was no reply, he turned back, eyebrows raised at the down cast and impossibly sinful sad eyes the cat was looking at him with.

Giving in like the weak man he was becoming, he approached the feline and gently pulled him into his arms, stroking through the unnaturally soft green fur, "don't know how huh?"
Catzuku whined, claws digging slightly into his shoulder in distress.
Shinsou turned back to pour the boiling water into the mug, stirring it to dissolve the coffee grains with one hand before settling at the kitchen table with Izuku in his arms.
"I call you kitten as a pet name..." he muttered, "yes I love it when you wear cat ears but this is taking it way too far."

The cat burrowed slightly more into his body and he scowled. "Aizawa could fix this right?"
The cat seemed to slump even more, and then he heard the faint wall shaking.
"Busy I guess." He muttered, feeling slightly queasy. "Have you tried thinking it?" He asked only to grimace as a claw dug sharply into his shoulder blade. "Sorry! Quit that! Yeah, I just don't see what else I can suggest or do!"

Catzuku pushed back to sit in his lap, and his eyes got steadily bigger and bigger, and rounder and warmer.
"I'll do whatever you want! Forever!" Yelled Shinsou automatically, "tell me! I'll do it!"

Can cats smirk?
The green feline dug its head under his hand and looked at him expectantly.
"Petting?" He asked dryly, "you seem trapped as a cat...and you just want me to pet you?"
Catzawa nodded, and preened as he resumed his ministrations, taking a sip of coffee as he artfully turned Izuku to liquid with one practiced hand. He knew all cat weak spots, and though it felt a weird level of intimate, he was too tired to over think the places on Izuku's body he was actually touching.

At some point he carried Izuku back upstairs to his room. He didn't want any classmates seeing them, and didn't particularly want to involve Shoto or Katsuki either. Izuku was a cat. This was his.

The cat in question stretched on his lap, looking at him so lovingly it made Shinsou shiver. And part of his brain still confused between cat and human was decided. Cats never looked this loving.

Then something else happened. The cat grew until Izuku was on his lap. Naked and on his lap.
He gazed determinately into the shorter teen's eyes. "You did it."
Seemingly unphased by his lack of clothes, Izuku leaned closer til they were nose to nose. "I wanted to kiss you. Couldn't do that as a cat."
The greenette then leaned in, his slim arms resting on the taller teen's shoulders as he kissed him slowly, lazily, hungrily.

When he pulled away, Shinsou chuckled, "you could turn back the whole time, couldn't you?"
"I can use quirks relatively instinctively as long as I understand their process. It took ages to work out how to turn cat, but it's the same experience to turn back."
"Then why?" He murmured, his hands moving to the greenette's waist and tracing his sides.
Izuku shivered at the touch, "missed you. Haven't been alone together in ages."

Shinsou sighed. It was true, the work load had been getting to him, and he hadn't spent as much time was he would have liked with Izuku. "I'm sorry Zu." He hummed, and leaned in to kiss him again before blinking in surprise. "You still have ears."
Devious green eyes glinted at him in the low lit room, and a furry appendage caressed his bare calf before flicking around behind Izuku. "I've a tail too."
Shinsou growled in spite of himself, "you only had to ask, you know."
Izuku giggled as the taller teen kissed his neck, "this was more fun, the look on your face...." he flushed a little as Shinsou's lips dipped lower and he shivered, "and the way your hands felt..."

Those very hands trailed slowly down to his hips, one finding and stroking his tail. Izuku moaned a little.
Shinsou's head raised to his left ear, "be good for me kitten."
Izuku purred.

———

Comic credits to moi.