Kannonzaki is the boyfriend of the protagonist, Haruna Wakakusa, and serves as a primary antagonist within the social circle at the center of River's Edge. He is characterized as a hotheaded, rash, and volatile young man, often acting as a schoolyard bully. His aggression is particularly and relentlessly targeted at a classmate named Ichiro Yamada, whom he torments out of jealousy over Ichiro's popularity with the other female students.

Kannonzaki comes from a broken home, being the child of divorced parents, and his violent behavior is presented as a maladaptive coping mechanism for his own deep-seated emotional pain and feelings of rejection. He deals with his emotional blocks by asserting dominance through physical force, creating a persona of an unprincipled rogue to mask his insecurity. Despite being Haruna's boyfriend, the relationship is portrayed as lazy and based more on convenience than genuine affection; Haruna initially slept with him once, after which he began calling himself her boyfriend, though she does not seem to genuinely like him. His mistreatment of Ichiro is a central point of friction in this relationship, as Haruna frequently intervenes to protect Ichiro from Kannonzaki's abuse.

His role in the story is largely that of an instigator of conflict, embodying the nihilistic and directionless cruelty that pervades the teenagers' environment. He is also involved in a separate, emotionless sexual relationship with Haruna's friend, Rumi, which further illustrates his pattern of seeking physical sensation as a substitute for authentic connection. His motivations are rooted in a need to feel powerful and in control, a desperate attempt to feel alive in the face of a reality he finds bleak and unrewarding. While he does not undergo a significant redemptive transformation, his influence over Haruna's life eventually wanes, and the narrative ultimately moves on from his toxic behavior. Other than his physical aggression and his status as a delinquent, Kannonzaki has no notable special abilities.