Ayachi Nene in her Witch form.
In Sanoba Witch, the witch is a person who formed a contract with an alp in order to have their wish granted. The price of a witch contract varies, and in Nene's case it's to wear a really revealing witch outfit and having random, involuntary fits of sexual arousal, with no care for time or place.
Due to the extreme sexiness of Nene's witch costume (and her equally extreme embarrassment), it's become the most popular Cosplay material for artists drawing characters from Yuzusoft's universe.