Kiriko Mizuki is the twin sister of Ryoko Mizuki and an investigator for Interpol. Her sister had infiltrated the secret evil organization known as G.O.D. for an undercover investigation, a decision that prevented the sisters from seeing each other for a long period. Despite this physical separation, Kiriko maintained secret contact with Ryoko. Through these covert communications, Ryoko requested that Kiriko provide assistance to Keisuke Jin, the young man who had been transformed into Kamen Rider X and had begun his fight against G.O.D..

Answering her sister's call, Kiriko steps forward to support Keisuke, as well as Tobei Tachibana, who later becomes one of his primary allies. However, she initially keeps her familial connection to Ryoko a secret. This decision, along with her sudden appearances, leads to confusion and uncertainty for Keisuke, who does not know whether to trust her. In one notable encounter, she sternly redirects Keisuke's focus from his personal turmoil to his duty as a hero, reminding him that protecting the citizens is his mission as X-Rider. This moment reveals her pragmatic and mission-driven personality, as she prioritizes the greater good over individual feelings.

Kiriko's motivations are rooted in loyalty to her sister and a sense of justice that compels her to fight against the G.O.D. organization, even at great personal risk. Her actions demonstrate a selfless dedication to the cause, as she is willing to put herself in danger to aid Keisuke. This courage culminates in her ultimate sacrifice. When G.O.D. planned to seize energy resources from Mount Kurogane, Kiriko accompanied Tachibana and Keisuke to protect a survey team. During this confrontation, she was questioned by Keisuke and finally revealed the truth that Ryoko was her twin sister. Shortly after, an arrow fired by the monster Strong Arms Atlas was aimed at Keisuke. Kiriko did not hesitate to throw herself in its path, protecting him and losing her life in the process.