Yuki Momose operates as one of the Three Heads under the Otherworld King’s command in the initial storyline, alongside Leon and Pantera. As a white card battler and core soldier bearing the light of the white core, his actions are driven by a singular goal: protecting his younger sister, Kajitsu Momose, who suffers under the immense burden of Mother Core’s power. This devotion traces back to their past lives, where Yuki reigned as king of the white world and Kajitsu was the princess of the green world, destined for marriage before their deaths led to reincarnation as siblings.

Yuki exhibits a cold, ruthless demeanor in service to the Otherworld King, executing morally ambiguous tasks to ensure Kajitsu’s survival. His loyalty fractures around episode 36 when he recognizes his methods intensify her suffering. Defecting from the antagonists, he leaves Kajitsu with the core soldiers and allies with them, swapping his white villain attire for a black hero outfit.

Their bond carries tragic weight—reincarnated lovers now siblings, further strained by Kajitsu’s deteriorating health and eventual death. The cycle persists into future lives: Kajitsu reincarnates as the mazoku Flora, while Yuki becomes the human Zolder, perpetuating their star-crossed dynamic with mismatched lifespans.

In the sequel series *Brave*, Yuki’s reincarnation Zolder emerges, inheriting his protective instincts toward Flora (Kajitsu’s reincarnation) and ultimately partnering with her. Throughout his journey, Yuki stands as Dan Bashin’s primary rival, engaging him in strategic card battles while wrestling with divided loyalties and desires. His abilities center on white-attribute cards, though specific mechanics remain undefined. His arc concludes with his and Kajitsu’s legacy enduring through Zolder and Flora in later series installments.