Serpent

Serpent was a criminal who killed Bartman's parents in a story told by Bart Simpson.

Biography
Bartman and his parents attended an opera at the Gotham City Opera House, and Serpent shot and killed Bartman's parents as the three of them were walking home. Bartman then decided to become a superhero, and began to fight crime for the purpose of finding the man who killed his parents. The only thing Bartman knew about the man who killed his parents was that he had a tattoo of a snake. Because Serpent wore a disguise, Bartman did not know his identity, so every time he apprehended a criminal, he checked the criminal for a snake tattoo.

Bartman later encountered Serpent after a robbery but failed to catch him. From the blimp he was using as a getaway vehicle, Serpent thanked Bartman for eliminating his criminal competitors and promised Bartman that he would get him next. Later when Serpent robbed the Gotham Natural History Museum, Bartman succeeded in arresting and unmasking him. Bartman saw the snake tattoo and realized that it was Serpent who killed his parents, and he then became enraged and killed him for revenge.

Behind the Laughter

 * Unlike the rest of Bartman's enemies, he doesn't appear to be based on any particular Batman villain.
 * However the "S" on his chest resembles the question mark worn by.
 * Also, being named after an animal may reference.