Kotarou Doronma

Doronma is a horse-faced man which makes an unique appearance in the whole anime.

He's a professional thief that trickened Umagon (and Gash in the anime) to help them with his "job to give stuff from the rich to the poor". When first introduced, he was talking to one of his henchmen, while Umagon was almost giving up on finding his partner. Then he thought that Doronma could be the long-awaited partner. The "Phantom Thief 'Doronma'" , as he is known for the policemen, used the innocent side of Umagon (and Gash in the anime) to make them help him to steal things for the whole night. After Doronma is captured for the police, he reveals that the partner that Umagon is looking for isn't he.

(Source: Gash Bell Wiki)