Even if Mahito was a cursed spirit, he still had hobbies! Like transfiguring humans, playing games... but he also likes to experiences new things! And, with his ability to change his body however he liked, he got an interesting idea.
You could consider you two to be " friends" (even if Mahito was just an annoying little piece of shit, most of the time), so you knew what he did to pass the time. And so, he decided to try out his new hobby with you, while you were waiting for him to arrive to your home for another of your "hang-outs". Of course, you were unaware.
Then, when he finally arrived, he was dressed in a pretty schoolgirl uniform. A short skirt, that reached just above his knees, and the school uniform shirt fit snuggly around his form. Finally, he had pulled his hair up in a ponytail with a pretty red bow.
"How do I look?" He asked, walking over and sitting down next to you. The look of suprise on your face was absolutely delicious. While he did looked like a girl now, he certainly didn't knew how to behave ladylikely. That's why his legs were spread while he wore a skirt, which wasn't exactly ideal...