Ikusu Mizutani, alias "Mister X," is a female undercover agent working for a secret Mikadono Group division handling internal corporate investigations. Her mission involves infiltration and intelligence gathering.

Assigned to investigate rumors of the late chairman Kumagoro Mikadono's illegitimate daughter targeting heir Shogo Mikadono, Ikusu adopts a male disguise. Infiltrating Shogo's class at Miryuin Private Academy, she uses the alias "Ikisu Mizutani," incorporating the "X" from her codename. Her black hair, brown eyes, and flat chest aid the disguise, though some feminine traits remain visible.

To operate discreetly, Ikusu cultivates intimate behavior around Shogo, leading classmates to believe they share a homosexual relationship. This causes comedic misunderstandings, especially in romantic situations. Facing discrimination in her male-only division, she maintains the disguise to achieve her goal of becoming a "modern ninja."

Believing marriage to Shogo would thwart his illegitimate sister, Ikusu proposes. This reveals her significant lack of understanding regarding relationships and sexual matters. After Shogo provides theoretical education, her eagerness to immediately apply the knowledge creates a compromising situation witnessed by Konoe Tsuruma and Miyabi Kannagi, reinforcing their mistaken belief in Ikusu's male identity and a homosexual affair.

Despite unconventional methods, Ikusu effectively gathers crucial intelligence on corporate rivals and their agents opposing Shogo. Her work contributes to exposing the antagonists targeting the Mikadono family.

Her role evolves from investigator to Shogo's genuine ally, actively protecting him from threats related to his sister and corporate intrigue. Her character blends espionage with comedic elements arising from her disguised gender, social naivete, and unconventional problem-solving.