Wilhelm is the first major boss of the game. (Major bosses are indicated by health bars on the HUD instead of above the enemy.) He is mentioned multiple times by the Guardian Angel, with considerable trepidation, before he is fought at the End of the Line.
Physically he is part Hyperion robot, with his body from the waist down replaced with loader legs. From waist up Wilhelm resembles an Engineer with a Constructor's eye in the place of his right eye. Wilhelm also bears a shoulder-mounted mortar, which he uses for ranged attacks.
An unused ECHO recording from Handsome Jack exists in which Jack says that he poisoned Wilhelm prior to his fight with the Vault Hunters, weakening him in order to ensure the power core he carried would be recovered without making the find seem too easy.