Izumi is one of the three girls who get hired by Yoshitaka. Izumi never uses an honorific suffix (-san, -sama, -kun etc.) after Yoshitaka's name. While this usually means the speaker is very close the person, this could also mean a grave insult. Izumi uses it to show her contempt and her complete lack of respect she has for him. Izumi was originally going to be the "normal" character of the series but she turned out to show violent tendencies towards Yoshitaka and became the "doll" of the household: Everyone, including Mitsuki, messes with her in some sort of sexual tendency much to Izumi's dread. Be it flipping her skirt or attempting to remove her bra, Izumi is constantly being harassed.
He is My Master
