Banjoe Onizuka begins as a notorious delinquent student at Aibo Academy, defined by his aggressive nature and bullying tendencies. He constantly seeks strength through street fights and Buddyfighting, earning a well-established reputation as a troublemaker, with peers like Ranma noting his habit of intimidating others. His character undergoes a pivotal shift during the ABC Cup tournament after losing his opening match to Masato Rikuo. Taking Masato's advice, he channels his energy exclusively into Buddyfighting, abandoning his previous violent behavior. He becomes more approachable, willingly accepts challenges, and seeks guidance to improve, gaining popularity that even overwhelms his buddy, Brawlzeus.

Following a second defeat against Masato, Banjoe experiences a period of emptiness and lost motivation. Yuga Mikado intervenes, reigniting his fighting spirit and prompting him to challenge Yuga in the Shop Preliminaries. Though he loses this match, he accepts the outcome gracefully and recommits to his ultimate goal: becoming the world's strongest man. Banjoe utilizes a Danger World deck focused on the Godpunk attribute, specializing in mechanics that plunder opponents' resources through card effects and costs. His item card is a Jumper, worn normally, contrasting with his typical delinquent style of draping his jacket like a coat cape.

His physical appearance, featuring bandages, a headband, and piercings, reflects his delinquent origins. Banjoe's development arc transforms him from an aggressive bully into a more humble and determined Buddyfighter who values growth through competition. Key episodes showcasing his journey include Episode 5 (his first battle against Masato) and Episode 21 (his fight against Yuga after overcoming his motivational slump).