Jun Kenzaki is a boxing prodigy and the main rival of Ryuuji Takane. He sustains a significant injury during his first fight with Ryuuji but continues boxing, determined to become the world's best despite this setback. His serious demeanor and determination define his approach. As part of Team Japan, he competes internationally, including against Team USA in the "Pacific War" arc. He fights N.B. Forrest, delivering a decisive uppercut. Later, during the Junior World Tournament, he confronts Führer Skorpion of Team Germany, unleashing his signature superblow, the Galactica Phantom, to win. His punching power is depicted as extraordinary, capable of crushing atoms.

Transitioning to professional boxing after the Junior World Championship, Kenzaki challenges reigning world champion Jesus J. Christ at age 17. Before this title fight, where Jesus employs the Neo-Bible technique, Kenzaki overcomes a confrontation with Ishimatsu Katori about his distant behavior toward Kiku Takane. Kenzaki wins and credits Katori for reigniting his fighting spirit. He then defeats Jesus J. Christ to become world champion.

His relationship with Kiku Takane evolves, culminating in marriage. On their wedding day, with Kiku wearing her mother's dress, Kenzaki issues a final challenge to Ryuuji. Their brutal match pushes both to their physical limits, resulting in Ryuuji's victory but leaving both near death. Determined to show Kiku the winner, Kenzaki assists the incapacitated Ryuuji to the church. Ryuuji collapses and dies in Kiku's arms upon arrival, while Kenzaki looks on with a proud smile.

In sequel media, Kenzaki and Kiku have a son named Rindo Kenzaki. Following their deaths, Rindo is raised by Ishimatsu Katori.