Natsuki Mogi begins as Takumi Fujiwara's high school classmate and love interest. After a period of limited contact, she reconnects with Takumi during summer break, inviting him on a beach date and stating her dislike for "boring guys who only like cars." This prompts Takumi to borrow his father's Toyota Sprinter Trueno AE86 for their outing, secured through his agreement to race Keisuke Takahashi.

Her past includes a relationship with senior soccer club member Miki, which ended after Takumi punched Miki for boasting in the locker room about sexually exploiting her. Natsuki later references this incident during a reconciliation with Takumi.

A defining element involves her secret engagement in compensated dating with an older man known as "Papa," who drives a Mercedes-Benz. This arrangement finances her lifestyle but causes significant conflict when Takumi discovers it, particularly after seeing her with the man near a love hotel. The revelation strains their relationship and contributes to Takumi's emotional distress during a crucial racing period.

In Legend 3: Dream, Natsuki confronts the unresolved issue of Takumi punching Miki, apologizing for not explaining sooner. This leads to a moment of reconciliation where they kiss. She subsequently moves to Tokyo for studies.

Following her departure, Takumi forms a new romantic relationship with golf prodigy Mika, concluding his connection with Natsuki. Her final appearances in later installments are limited to brief cameos without substantial narrative impact.