In Mega Man Zero, the false ruler of Neo Arcadia is Copy X.
Copy X is a replica of the original Mega Man X, created to govern Neo Arcadia after the Elf Wars. Unlike the heroic X, Copy X presides over a regime that persecutes Reploids during an energy shortage. His government presents Neo Arcadia as a peaceful utopia, but its policies include the destruction of innocent Reploids to conserve resources.
Zero is awakened by Ciel and joins the Resistance against Neo Arcadia. After defeating the Four Guardians, he confronts Copy X and defeats him. Copy X self-destructs rather than accept defeat, making him the first game's principal antagonist. The original X does not appear as the ruler; the story specifically distinguishes the real hero from his imperfect copy.
Copy X is also easy to confuse with Elpizo, Omega, or Dr. Weil. Those characters become important later in the series, but none is the false ruler introduced in the first Mega Man Zero game.