It's Izuku: he freaks the hell out.

"...AAAAAAAAAAAAAAAHHHHHH!" Izuku shrieked as he tried and failed to grasp what was going on.

"... Master's a bit of a screamer, isn't he?" Momo asked, watching in bemusement as Izuku started running through things in his head and aloud. "...hopefully he can make one out of me..."

"A genie!? An actual genie!? No, no, that's not possible, genies and magic don't exist!" Izuku was muttering rapidly as he tried to rationalize it all. "Some kind of Quirk based on the concept, maybe? Yeah, that makes more sense, but in that case, how does it work? Can she literally bend reality to some extent to grant wishes? How many wishes are involved, the standard three?" He continued off on his tangent.

Momo watched patiently for a solid ten minutes as he tried to figure it out. Normally she would have said something, but as a genie, she was more inclined to just let him have his moment until she was permitted to explain. "Done, Master?" she asked, once he had finally run out of ideas on how the Quirk might work.

"...yeah, I guess I am," Izuku admitted sheepishly, surprised she waited that long. "...so, um, how does your Quirk work?"

"My Quirk? If you are referring to the fact that I am a genie, it's not a Quirk. My actual Quirk allows me to create objects from my imagination and lipids, so long as I can visualize and understand them well enough. The genie thing is actually magic. Or at least cosmic energy that can tamper with the fabric of reality, with some limits, if you want to get a little more technical. Magic is a bit of an oversimplification of it all, honestly. It has scientific explanations, just well beyond the grasp of easily explained physics. Might be best to just stick with saying it's magic, it might fry someone's brain to try to fully understand it," Momo explained. "This lamp came from a dimension well beyond our own, and made me into a genie bound to it until I've filled a certain quota."

"That can't be right... It's not possible!" Izuku said.

"Perhaps you would like to test it to be certain, Master?" Momo suggested.

"...you mean, make a wish?" Izuku asked.

"That's what I mean, yes," Momo replied, smiling serenely.