Helvi

Helvi is ultimate being (she is literally the Devil) as she says about herself. She was summoned by Theo Asper and stayed with him because of his wish. When he summons the devil Helvi he completely forgets his original goal (to gain power) and asks to be married to her instead. Helvi doesn't believe him at first, but sees in his heart that he is completely smitten with her (as him she isn't gonna share Theo to anyone). Helvi states that most of the times she has been summoned the wisher would then flee after getting their wish, trying to avoid paying the price. She didn't let any of them escape. She is wearing black short dress with stocking alongside Musketeers. She has black wings and horns (which turns into Ahoge - reminding her horns). Her hair is snow-white. Also she has white giant earrings. Helvi's personality reveal that she can barely hold herself from "assaulting" Theo (in good way). She also hold herself from doing what she would do otherwise - like killing someone in front of everybody. Despite being a devil, she cares for his husband and pass for other humans hiding her horns and wings. You could say she's quite Yandere.