>>531519298
She's rabbit girl with a design based on Nine the Phantom. This is what her ears look without the hat.
>>531521502
Venefica is one of the big characters. Her story is supposed to have more content down the line.
If you're curious, the other big characters are Sofia and Kotone, which you could probably guess. I think a fourth one was mentioned but not implemented, but I have no source to back that up.