Judai Yuki enters Duel Academy as an energetic, optimistic duelist with exceptional dueling talent but poor academic performance, resulting in Slifer Red dorm placement. His carefree nature and deep passion for dueling define his early character, alongside a strong bond with his cards—particularly "Winged Kuriboh," gifted by Yugi Mutou during his entrance exam. This connection manifests as an innate ability to perceive and communicate with Duel Monster spirits, linked to the "Gentle Darkness" within him.

Traumatic experiences in an alternate dimension catalyze profound change. Witnessing friends' apparent deaths and subsequent possession by his past life’s spirit, the Supreme King, instills guilt and horror over actions committed while possessed. His personality shifts drastically: he becomes withdrawn, cynical, and loses enjoyment of dueling, distancing himself from friends to shield them from dangers tied to his presence. Aster Phoenix’s sacrifice and revelations about Yubel’s manipulations partially restore his resolve. He ultimately uses "Super Polymerization" to fuse his soul with Yubel, his childhood guardian spirit, purging her corruption and gaining new abilities.

This fusion fundamentally alters his nature, blending human and Duel Monster spirit. He gains powers including sensing thoughts and emotions, seeing future visions, resisting supernatural influences like hypnosis, physically manifesting Duel Monsters, and enhanced athleticism. Entities like Nightshroud perceive him as a non-human hybrid. Initial emotional detachment and seriousness in the fourth season gradually give way to rekindled optimism through interactions with friends and a pivotal duel against Yugi Mutou, restoring his love for dueling.

In "Yu-Gi-Oh! 3D: Bonds Beyond Time," set after graduation and his duel with Yugi, Judai operates as an independent duelist investigating mysterious card disappearances in Venice. He retains all abilities from his fusion with Yubel and the Supreme King. When attacked by Paradox using corrupted monsters like "Cyber End Dragon" and "Rainbow Dragon," Judai summons "Elemental HERO Neos" for defense. Yusei Fudo and the Crimson Dragon rescue him from Paradox’s "Stardust Dragon." Judai expresses excitement about Yusei’s Duel Runner technology. After confirming Paradox’s threat through historical records and experiencing the erasure of his "Elemental HERO Neos" card due to temporal disruption, he allies with Yusei and Yugi Mutou.

During the trio’s duel against Paradox, Judai coordinates strategies, showcasing signature fusion tactics. He borrows Yusei’s "Junk Gardna" to fuse with "Elemental HERO Neos," summoning "Elemental HERO Neos Knight" and utilizing its multi-attack ability. Anger at seeing Jesse Andersen’s "Rainbow Dragon" corrupted reflects his enduring loyalty. His catchphrases and energetic demeanor confirm the recovery of his original passionate personality.