Gozaru

Mimi is a slut.

Just look at her, if you look at the relationship she has with her family, sexual tension is abundant, so she was probably raped by her father or someone.
Because she was raped at a young age, she appears to have bent herself to try and enjoy it, reflecting what sometimes happens in real life situations. This explains the lack of any real resistance when she is molested by Rin, especially the time she had her breasts sucked forcefully. This submissiveness is
present because her experiences with her father, and possibly father's friends.

Another telling factor is her attraction towards older men, particularly Rin's first cousin, once removed, and guardian Reiji. This attraction is bred by her repeated rapings, and how she came to enjoy and like them. Because of this, she is sexually attracted towards forceful, 'dirty' adults. Reiji fits this position well; he is raising Rin according to some kind of Hikaru Genji plan, and has some very dirty thoughts thinly veiled under the guise of a "loving guardian". Mimi is no idiot, she just acts innocent to further her own agenda, and can see right through Keiji.

All of these point directly to the fact that the "cute and innocent" Mimi is nothing but a facade, disguising her horrifying true self, a sex-starved slut lusting for a man to rape her.