Kazumi Onimaru emerges as a central antagonist in the series’ fourth season, reigning as the undefeated "Under 20" Vanguard champion. His abrupt introduction occurs during an unexpected clash at a card shop, where he overwhelms protagonist Chrono Shindou using a Nubatama clan deck. This decisive victory cements his dominance within the competitive circuit and spurs Chrono to recruit allies for the looming U20 Championship.

Kazumi’s striking appearance features long blond hair pulled into a ponytail, piercing yellow eyes, and a sharp black suit paired with a crimson shirt. His likeness to Kouji Ibuki—a resemblance Chrono openly acknowledges—hints at deeper ties to the narrative’s lore. As a diffriden bound to Stealth Dragon, Shiranui, Kazumi operates among antagonists seeking to dismantle the bond between Earth and Cray by exploiting the U20 tournament’s stakes.

His calculated campaign targets pivotal fighters, overpowering Team TRY3 members and rivals such as Am Chouno, Luna Yumizuki, and Kumi Okazaki. Each triumph amplifies his menace, forcing Chrono, Shion, and Tokoha to forge fresh alliances and sharpen their strategies. Kazumi’s relentless victories heighten narrative stakes, positioning him as both a tactical obstacle and a catalyst for the protagonists’ evolution throughout the championship’s escalating conflicts.