Ikusu Mizutani is a character from the anime NAKAIMO - My Little Sister Is Among Them!. Originally introduced under the codename Mister X, she is a female agent belonging to the Seiryu Association, a secretive organization that handles internal investigations for the powerful Mikadono Group. Because the association is exclusively male, she maintains a male disguise to work within it, adopting the alias Ikusu Mizutani as a play on the X in her codename. Her long black hair is typically tied back and hidden under a cabbie hat as part of this disguise, and her naturally flat chest aids in passing as a boy, although she possesses a face and voice that are notably beautiful for a girl.

In her public persona as a male student at Miryuin Private Academy, Ikusu comes across as clever, charming, and somewhat mysterious. However, this contrasts sharply with her private personality, which is that of a playful and carefree tomboy with a weak sense of personal boundaries. She greatly enjoys teasing Shogo Mikadono, the protagonist, frequently creating awkward and compromising situations between him and the other heroines for her own amusement. Her boldness extends to a notable lack of understanding regarding social norms and personal privacy, such as when she uses Shogo's apartment shower as part of her daily routine without invitation.

Ikusu is driven by her goal of becoming a modern-day ninja, a dream that requires her to operate within the men-only Seiryu Association and thus maintain her male disguise. Her official mission is to investigate rumors of an illegitimate sister of Shogo who is attempting to enter his life, possibly to claim a share of the family inheritance. As the story progresses, her methods become increasingly unconventional. In a sincere but misguided effort to protect Shogo by making him unavailable for marriage to his sister, she proposes to him directly. This action reveals an extreme naivete about romance and intimate relationships, leading Shogo to give her a theoretical lesson on the subject, which she then eagerly and inappropriately attempts to put into practice.

Her relationship with Shogo is the central axis of her role in the story. As one of the few people who knows her true gender, he is frequently the target of her teasing and the unwitting participant in her schemes. At school, her familiar behavior around Shogo leads their classmates to mistakenly believe that the two boys are in a homosexual relationship. Despite the comedic nature of their interactions and her poor understanding of social etiquette, her role evolves significantly over the course of the series. She transitions from being a detached investigator merely following orders to a genuine and loyal ally, actively using her skills to protect Shogo from real threats posed by corporate rivals and other agents. Her primary notable ability lies in her espionage and infiltration training, which allows her to maintain a complex disguise, gather intelligence, and conduct surveillance. Her work proves crucial in uncovering information about the true nature of the threats surrounding Shogo and the Mikadono family.