Heart is one of the Four Chief Vassals of the King organization led by Shin, along with Spade, Diamond, and Club. A large fat man, Heart's stomach was composed of a rubber-like substance which absorbs the impact of most martial art techniques, earning him the nickname Kenpō Koroshi (拳法殺し, The Destroyer of Fists). While normally calm and more merciful than his fellow Kingsmen, his personality changes to that of a homicidal maniac at the sight of his own blood.

(Source: Wikia)