Kain R. Heinlein (カイン・Rアル・ハインライン) is a character in the Fatal Fury series. While he is the final boss of Garou: Mark of the Wolves, he will only fight players who've attained at least an AAA-ranking win average throughout the game and never lose a single round. He is the younger brother of Marie Heinlein, the brother-in-law of Geese Howard, and also the maternal uncle of Rock Howard.