Ruka, also recognized as Luna in certain regions, is the fraternal twin of Leo and a Signer marked by the Crimson Dragon’s front claw. A month-long childhood coma stranded her consciousness in the Duel Monsters Spirit World under Ancient Fairy Dragon’s guardianship, forging her bond with spirits such as Kuribon and Regulus. This link granted her sensitivity to looming threats and the emotional echoes of others’ Decks, including detecting anguish within Akiza Izinski’s Plant Deck during the Fortune Cup.

Her physical fragility, rooted in post-coma recovery, restricted her early activities, leaving her drained after Duels and often housebound. Gradual resilience emerged post-Dark Signers conflict, though susceptibility to injury lingered. At the Fortune Cup, Leo’s attempt to impersonate her was overturned when her Signer identity solidified. Amid the Dark Signers crisis, she intervened to liberate spirits from Zeman the Ape King’s tyranny and thwarted Sayer’s schemes at the Arcadia Movement.

Manga iterations depict stark contrasts: post-Duel Dragon ceremony, she grows distant and emotionally muted, channeling expressions through a Kuribon plush. Shadow Miasma from Ancient Pixie Dragon briefly amplified aggression, manifesting in controlling Leo and callousness toward her Duel Dragon. These traits dissolved after her rematch defeat against Yusei Fudo, reverting to her anime counterpart’s demeanor.

Within *Yu-Gi-Oh! 3D: Bonds Beyond Time*, her collaboration with Leo and Akiza supplied critical historical insights for Yusei’s inquiry into Paradox’s temporal disruptions. Their research clarified Yugi Muto’s legacy and Paradox’s meddling across eras.

Post-Dark Signers, she joined Duel Academy with Leo, donning standard uniforms and supporting Team 5D’s WRGP efforts as a pit crew member. A later timeskip portrays her collegiate years, marked by elongated hair and refined attire, signaling her evolution from fragility into poised maturity.