Zakira Fua is the main villain pulling strings across multiple generations of the series. He's the one who's been controlling his younger sister Yuu from the beginning, and he serves as the primary antagonist for both Shori and Shobu.
As the leader of the Fua Duelists, Zakira holds the top rank (marked as Z, ranked 1st) within the organization. He's best known for his ruthless dueling style and his manipulative nature—traits that define his role throughout the story.
The timeline stuff gets a bit messy with Zakira. In Versus Revolution, he shows up again during Katta Kirifuda's era (Shori's son and Shobu's younger brother) through Basara and a card from the past, though only as an image rather than in person. Interestingly, this creates a plot hole since Zakira doesn't actually appear in Duel Masters Charge, Duel Masters Fighting Edge, or Star Cross—the three manga where his operations in Shobu's time supposedly happen.
Content compiled by AnimeList.moe from publicly available sources.