Natsuki Mogi is a high school student involved with Takumi Fujiwara during the Akina's Eight-Six events. She previously managed her school's soccer club and had a boyfriend; Takumi punched this ex-boyfriend over inappropriate remarks about her, causing a temporary rift in their friendship until Natsuki learned Takumi's reason.

She secretly maintained a transactional relationship with an older, married salaryman driving a white Mercedes, accepting payment initially for companionship and later for sexual encounters. This man is later revealed as the father of her friend Shiraishi. Natsuki initially justified the arrangement but grew increasingly uncomfortable with it.

Her relationship with Takumi deepens during shared outings to Ikaho Onsen, Lake Akina, and Kashiwazaki beach. She observes his smooth driving prevents her motion sickness and encourages him to share his feelings and racing struggles. At Lake Akina, they share their first kiss after she apologizes for misjudging his fight with her ex-boyfriend. She expresses concern for his racing safety and supports his dream of owning a car, playfully suggesting she could fund it.

Her secret relationship becomes a major conflict. Developing genuine feelings for Takumi, she ends the transactional arrangement, seeking emotional connection beyond financial exchange. The salaryman requests a final meeting for closure, which Takumi witnesses, leading to an angry confrontation and Takumi ignoring her. Confused and hurt by his rejection, she replays the confrontation emotionally.

Weeks later, she attempts reconciliation, directly asking Takumi if they remain friends. She arranges another meeting at Lake Akina to explain she ended the affair and offers a sincere apology, believing his anger stems from betrayal and lingering affection. Despite her efforts, their relationship remains strained.

Following graduation, she plans to attend university in Tokyo, citing a desire for independence from her parents.