Mizuho Asano

Keigo's formidable older sister, Mizuho regularly sends Keigo on errands (like getting juice from a vending machine) with the usual punishment for lateness being a kick to the crotch. She happily welcomes Ikkaku and Yumichika into the Asano home after Keigo brings them back. While she initially claims that she is welcoming them "because that is the kind of sister" she wants to be, in reality she just has a thing for shaved/bald heads. She has a tendency to speak sexily and show off her figure to men she finds attractive, much to Keigo's chagrin.

Source: Wikipedia (