Ikusu Mizutani (水谷 衣楠/Mizutani Ikusu) is one of the heroines of the Kono Naka ni Hitori, Imouto ga Iru! series.
Ikusu Mizutani switches between her disguises daily, with the most notable one being her male persona in which she enrolls in the Miryuin Academy.
In public, Mizutani appears as a clever and charming person, with an air of mystery. This appearance is best seen while she disguised as a man. In private, however, she has little idea of privacy and is carefree to a fault.
She also enjoys teasing Shougo in many ways, most commonly by pushing him into situations which causes misunderstanding between him, Konoe and Miabi. She even causes rumours that Shougo is a homosexual and is dating Ikusu.
A member of the Seiryu Association. Initially introduced as "Mister X", she was sent to investigate on the identity of Shogo's younger sister while disguising herself as a male student in Miryuin Private Academy. She often sneaks into Shogo's apartment to use his shower as part of her routine. She is trying to become a modern day ninja, and as the Seiryu Association is men-only, she disguised herself as a guy to become part of the association. She often puts Shogo in awkward situations.