Dake-san

ダケさん

Dake-san is summoned to put an end to Mitani's cheating in Go. Initially, he plays using his weaker hand and deliberately makes poor moves. He then becomes serious, causing Mitani to learn his lesson (and lose all his money).
He is a very strong player, and it was stated that he did not need to cheat to defeat Mitani, but did so merely to make a point.