Seiji Kido emerges as an antagonist in season 6. Introduced by Subaru Hoshiyomi in episode 242, he is a chess prodigy. His partner is the Drametal attribute monster Lostknight: Nero Lance from the Star Dragon World.

Kido wields a strategic Drametal deck specializing in area control and oppression. A signature spell card, "Seiji Kido Defense," embodies his tactics. This card protects a Drametal monster on an oppressed area from destruction for the turn or retrieves a Drametal monster from his drop zone. His strategy manipulates opponent fields through oppression, exemplified by cards like "Cho-Tokyo Gambit" and "Lostknight: Nero Lance," which moves to an opponent's field to inflict end-of-turn damage.

Vile Ranma targets Kido's skills for an alliance, culminating in a pivotal episode 244 Buddyfight where Subaru challenges Ranma. This clash directly decides Seiji Kido's freedom and Subaru's fate. Kido's own motivations include a plan to overthrow Ranma, revealed during episode 26. His competitive record features three documented matches against Subaru: a victory in episode A17 and defeats in episodes A26 and A38.