Kinnii

Kinnii, sometimes spelled as Kin'nee, is an anime-only character who volunteered to deal with people who had been abusing their Jusenkyo curses. He is considered a near invincible opponent, with his only weakness being that he fell into the Spring of Drowned Buddhist Priest (Fūshannīchuan), which causes Kinnii to lose all willingness to fight.

(Source: Ranma Wikia)