Bring Ochaco here, and make her a cat girl.

Izuku then decided to text Ochaco to come over, and looked at Momo. "Momo, I wish that when Ochaco arrives, she'll strip naked and then turn into a Neko."

"Oh, what kind?" Momo asked, raising her fingers to snap them, and Izuku began to explain.

"She should only have fur on her ears and tail, the ears transforming from her normal ones into cat ears and migrating up her head... Let's see... Oh, and make the ears and tail erogenously sensitive to the touch, and make it so her tail being grabbed will cause pleasure rather than pain. Next... Let's also make it so her tongue will become rougher, like a cat's, but not enough to irritate the skin she licks, simply enough to provide additional stimulation. And... Let's also have her voice box alter to allow her to purr and meow, while still retaining the ability for human speech. And maybe a slight compulsion to literally purr when happy, as well as when she's being sexually stimulated. And make her back mildly erogenously sensitive in that form too, so she can enjoy being pet like a cat while in it...

"Oh... And let's also change her mannerisms a little while she's a cat girl... as in, she'll have catlike mannerisms for the duration. Nothing major, just a slight compulsion to act more like a cat... And while we obviously can't make the change 24/7 for obvious reasons, I'd like for her to be able to switch back and forth, either at will, or automatically at my command... Oh, and let's make her feel more natural on all fours when she's a cat girl... Yeah, I think that will do it," Izuku nodded with finality.

"Very nice~!" Momo nodded, snapping her fingers, sending the spell Ochaco's way to trigger once she came in. "Previous Sex Genies have had Masters that just slapped on the cat ears and tail, and called it a day, but this is more fun~!"

"Wait, how did you know about previous genies?" Izuku asked.

"The lamp told me," Momo grinned. "I maintain a connection, and can essentially... Well, not quite view, but get a sense of what other genies did in service to their Masters..."

"Oh..." Izuku blinked. Momo proceeded to sit on his lap, wearing her harem outfit as she cuddled against him while they waited for Ochaco.

Before long, she arrived, and asked, "You called, Izuku?" She then blinked as the moment she closed the door behind her, her hands started moving on their own, and started undoing her clothes. "...oh, okay!" she giggled as her body disrobed without her input, realizing Izuku had something to do with it, and happily going along with it.

Upon dropping her last garment, she felt strange, yet familiar and pleasurable tingles along her spine, strongest at her tailbone, as well as feeling them in her her tongue, ears and throat. She moaned as she felt her tailbone start to expand, and fell to her knees, feeling a minor compulsion to do so she could have resisted, but chose not to. That same compulsion made her lean forward, falling onto her ears as well. Her ears grew more pointed and started to move to the top of her head, growing light brown fur with white tips.

The tingling in her back stopped first, followed by her tongue, and and then her throat. "Nyah!" she mewled on instinct, before blinking in surprise at the sound she let out, before she started purring as her tail grew out, brown furred with a white tip like her new ears. The transformation finished as her tail fully grew out, but she remained on all fours, feeling like it was natural. She looked up, her new tail swaying and her ears twitching. She meowed again, and beamed as Izuku, having watched with Momo, showed her a mirror.

"Nyah, I look adorable Master, thank you!" she mewed, cutely bringing up a hand to lick at the back of it, not even questioning why she did so. This was definitely a surprise, but not one she didn't find herself enjoying immensely. "You wanted a cute little kitten to play with, huh?" she asked in her best seductive voice, continuing to purr.

"I take it you like it?" he asked her, smiling. "It was just a crazy thought I wanted to try."

"I love it! It promises to be fun! Now you can really get some pussy, in more ways than one, Nyah!" she meowed in amusement. "In all honesty, I was wondering when you'd try these kind of sexy changes, Nyah..."