Yaezaki Erika

A spoiled rich girl. She refuses to have sex with Hiroki until he promises to be her slave, something he will never do. This constantly frustrates her. Her grandmother told her she is only to have relationships with the most upper class man that will devote his life to being her servant. If Hiroki won't agree to be her slave she's afraid what her grandmother will do to her.

Spoiler

When Hiroki makes Minako pregnant a second time she finally gives in and lets Hiroki have sex with her out of sheer frustration that she's being beaten by his own sister.