She is a classmate of Shiratori who uses her batting skills to punish Tsubasa for his womanising. She would normally use a customized baseball bat with spikes, but "because of the adults' circumstances", her bat was taken away and replaced with a plastic megaphone.

Shiratori Ryuushi is a young artist-in-training, about to start studying at a vocational school. Arriving at his new lodgings, Narutakisou, he finds himself the focus of attention for the collection of weirdos that inhabit the place. He also meets and falls for the landlady, Aoba Kozue, who seems to be both lovely and charming. However, Kozue is not what she seems to be... in fact, she's more.
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(Source: Anime News Network)

Mizuho Amane