Koizumi is a high school student who attends the same school as Kanna. She is initially portrayed as flighty and impulsive, often prioritizing following her favorite rock band over her schoolwork. Her decision to research the Bloody New Year's Eve incident for a history assignment draws her into the orbit of the Friend cult. Through her investigation, she meets Kamisama and learns the truth of that night, after which she is forcibly recruited into Tomodachi Land, a reeducation program where she must adhere to the cult's activities or risk being sent to Tomodachi World. She struggles to survive the experience until she meets Yoshitsune, who helps her pass the test in exchange for her help uncovering information about the identity of Friend. In the later timeline of 3 FE, she discovers an unexpected talent for bowling, which makes her heavily recruited by Kamisama. Koizumi serves as an accidental resistance member, acting as a key informant for Yoshitsune and a supportive friend to Kanna. Her personality is marked by optimism, common sense, bravery, and a sarcastic wit, which she uses as a shield against the cult's propaganda and psychological manipulation. She dislikes blind obedience and values independent thinkers. Though she is not a fighter, she survives intense ordeals including the imprisonment in Umihotaru and the events of the Bloody New Year's Eve. Her role in the story highlights how ordinary citizens can uncover extraordinary conspiracies. Over the course of the narrative, she develops from a carefree student into a committed participant in the resistance, driven by a sense of truth and justice. She views Yoshitsune as a reliable mentor and grows into a loyal friend to Kanna, with their bond strengthened through shared danger. Her relationships with the cult figures, particularly Kamisama and the Friend, reinforce her determination to stand against their ideologies. Her experience balances moments of levity with the seriousness of the cult's threats, making her an audience surrogate whose reactions underline the thriller elements of the story.