Nerisu

A demon girl from hell. She tells Takehara that she's also his true sister and that he needs to come with her to hell and become the next demon king. She fights the other sisters for possession of Takehara, but they're all witches, so it ends in a stalemate.

Instead of taking him by force, she agrees with the other sisters that if one of them can become pregnant, she won't take him to hell. But if she becomes pregnant herself...

During sex she can turn herself into an older version of herself with bigger boobs. As a demon, she loves breaking taboos so she also turns herself into a boy because the only thing more taboo than incest is homosexual incest.