Turns out she actually is a demon 💀
Though with WOoHS, we now know that she is in fact not a demon, but a "magical being" like Patchouli. So she is a magician? 🤔 But she's also like a familiar to Patchouli. So she's a slave who's also a witch?
If we home in on the magical being part, another way to look at it would be a homunculus. Patchouli created Koakuma just like how Keiki created Mayumi.