Zero, known fully as Rei Kanou, is an older junior high student due to academic setbacks. He endures classmates' mockery over his age and delinquent reputation, frequently skipping class and showing little interest in academics. His defining passion is motorcycles, embodying rebellious biker culture.

Initially aloof and selfish, Zero acts as a "Jerk with a Heart of Gold," using a rough exterior to conceal underlying kindness. His character evolves significantly through interactions with Jun, a younger classmate sharing his mechanical interest. While he first exploits Jun's affections for free meals at her family's harbor restaurant, he gradually develops genuine care and respect for her.

A key development sees Zero temporarily relocate to England. Returning, he initially adopts an overly serious, rigid persona, rejecting his past identity. This change draws disapproval, especially from Jun, and is later revealed as a temporary facade. His eventual return to his true self highlights his internal conflict and growth.

Zero's relationship with Jun shifts from one-sided affection to mutual romantic tension, culminating in a discreetly portrayed finale kiss. Despite this progression, practical challenges lead to a bittersweet resolution where Zero departs for England again. Jun entrusts him with her robot creation, Kichinosuke, during this parting.

Family influence comes through his supportive grandmother, who encourages his bond with Jun and recognizes their compatibility. Zero also navigates external conflicts, including rivalries with a violent motorcycle gang and a love triangle involving Yoko, an "Alpha Bitch" classmate who initially vies for his affection.