Manny Quinn is a secret character in Red Dead Revolver's singleplayer and multiplayer modes.
Background
Throughout the game, "Manny Quinn" is seen as an inanimate object in the background of the Brimstone gun shop. Presumably he is a dummy used for target practice.
Red Wood Revolver
Upon completing the game on Very Hard Mode, the player will unlock Red Wood Revolver Mode, which allows you to play as Manny Quinn in the story mode.