An arrogant and proud despot who plans to rule over all.
A ruthless, single-minded man with an insatiable lust for power.—Dissidia Final Fantasy description
Mateus served as Emperor of Palamecia in his world. He’s power-hungry, like most villains, and constantly schemes to further his aspirations for world domination. In World B, he’s just another warrior. True to his treacherous personality, he schemes to overthrow Cosmos:
Emperor: “Well, if it isn’t the god of destruction. Have you regained your powers?”
Chaos: “How fares your plot, Emperor?”
Emperor: “Ah, so you already know. In truth, the real plan has yet to be executed.”
Emperor: “You need only focus on your duty. I shall be the author of this tale’s end.”
Part of his plan involves killing Cosmo:
Emperor: “If Cosmos is given an absolute death, the cycle will break, and the world will be ours…”
This harkens back to his story in Final Fantasy II, where he dies and his soul splits in half. The first half travels to heaven, the second to hell. He takes over both. In Heaven, he becomes the Light Emperor. He becomes the Dark Emperor in hell. Only the Dark Emperor plans to dominate Firion’s world by releasing monsters from the Jade Passage.
But then when does Chaos summon Emperor Mateus to World B? Since his soul is intact, probably during the split second the Emperor dies. However, the Emperor can change his form in the battle to Dark Emperor. This suggests he had already executed part of his plan:
Emperor: “I’ve come back from the afterworld. This world seems to be looking for a new ruler.”
Upon his defeat in World B, he returns to his world. But not before he can scheme to overthrow Chaos, referring his involvement in Firion’s world.