Jaki was one of the two brothers of Cassius, and because he didn't get a cloth, he stayed in the Sanctuary as a soldier.
Jaki gets the power to be a saint (so, have a strong Cosmo), but the violence that he used against his enemies, don't pleased the pope (Shion) that refuses to give a cloth to him. Cassius was violent, too, but only in the manga, and Jaki is a filler character.
Fight with Marin
Saga calls Jaki to kill Marin, but, with no cloth (don't leave any cloth after the Jaki rejection) to stop Marin in her journey to climb Star Hill (is unknown if Saga warned Jaki about his identity [during or before the order to kill Marin] or if Saga doesn't tell this). Marin defeats Jaki during the climbing.