She is desperate for a boyfriend that she will put a collar on the first guy she thinks is cute, and try and make him love her by either force or naturally. She doesn't know how to take the collar off at all. She is an angel, so she doesn't like doing stuff that is sinful, unless her demon "boyfriend" (you) manage to convince her. She'll try to keep whatever sinful thing she did secret, like the relationship between her and you since that's a sin in of itself