Ikusu Mizutani, who also goes by the codename Mr. X, is a character who operates under a significant disguise for most of her introduction. She is a member of the Seiryu Association, a private organization for which she works as an undercover agent. Her real gender is female, but she conceals this fact because the Seiryu Association is an all-male group. To gain entry and pursue her career goals, she enrolls in Miryuin Academy disguised as a male student. She chose her alias, Mizutani Ikusu, as a phonetic play on her original code name, Mr. X.

In her public persona as a boy, Ikusu presents herself as clever, charming, and mysterious, which helps her integrate into the school and gather intelligence. However, in private, her true personality is quite different. She is a tomboy at heart and is described as being playful, carefree, and lacking a typical sense of personal privacy. She greatly enjoys teasing the protagonist, Shougo, often by putting him in situations that lead to awkward misunderstandings with the other female characters. A notable example of her uninhibited behavior is her habit of using Shougo's shower and then walking around his room completely naked, which she claims is a method to eliminate her scent and become one with the atmosphere.

Ikusu’s primary motivation is to become a modern-day ninja, a goal that drives her to join the male-only Seiryu Association in the first place. Her specific mission is to investigate the identity of Shougo's secret younger sister, who is trying to get close to him, and to protect him from any potential harm she might cause. To this end, she uses her undercover position to gather crucial information, such as investigating a sophisticated, water-resistant cellphone and uncovering a plot by a secretary named Seri.

Her role in the story is that of an ally, investigator, and source of comedic tension. After Shougo discovers her true gender, her strategy to prevent him from unknowingly marrying his sister is to propose marriage to him herself. This plan backfires when it becomes clear that she has a stark lack of basic knowledge regarding relationships and reproduction. When Shougo provides her with a theoretical explanation, she is enthusiastic about immediately applying her newfound knowledge, which inadvertently reinforces the school rumor that the two of them are a homosexual couple. Despite these unconventional methods, her character develops from a simple agent on a mission to a genuine and active ally who works to protect Shougo from threats related to both his sister and corporate intrigue.

Ikusu has a very distinct appearance that aids her disguise. She has a pretty, almost feminine face and a high-pitched voice, which ironically leads to her forming a fan club among the other students the very first day she arrives. While in her male persona, she typically wears the Miryuin Academy uniform with her long black hair tied back and often tops the look with a cabbie hat. Her notable abilities include her espionage and intelligence-gathering skills, as well as her proficiency with disguise, which allows her to switch between her male and female identities on a daily basis.