An antihero is a protagonist who plays the main role (or one of the main roles) in a story, but does not entirely fit the traditional archetype of a hero. Such characters may perform the deeds asked of them and save the day, but the manner in which they save the day may not serve the most noble of ends, nor are the characters themselves necessarily role models. In another context, an antihero might even be considered a villain; for example, a burglar that enters a residence with the intent to commit theft, only to stumble across and stop a murder in progress.
The first ever hero in literary history, Gilgamesh, may also be considered an anti-hero by today's standards.
Examples of Antiheroes
The protagonist of the Metal Gear series, Solid Snake is a trained soldier that for a time serves as a member of FOXHOUND before electing to retire after the Outer Heaven incident in the original Metal Gear. After being called back to service during the Zanzibar Land incident in Metal Gear 2: Solid Snake, he once again elects to retire to Alaska. Disillusioned with the military, he is again unwillingly dragged back into service in Metal Gear Solid when the current members of FOXHOUND rise up at Shadow Moses Island. He has little motivation to take part in the mission other than to learn more about Liquid Snake, the current leader of FOXHOUND that bears a striking resemblance to him. Though he was once young and idealistic, his experiences in the field have led to his becoming resistant to the desire to become involved save for those elements that matter personally to him.
The Grand Theft Auto series is rife with antihero protagonists. The characters the player controls in these games are typically criminals whose end goals are achieved through the committing of yet more crimes. Tommy Vercetti, the protagonist of Grand Theft Auto: Vice City, is one such character. He is a convicted felon who at the start of the game is released from prison after serving fifteen years for manslaughter, only to return to work for the Forelli family. He is sent to Vice City to expand the family's influence in the drug trade, only to run into trouble when a drug exchange goes poorly and he loses both the drugs and the Forellis' money.
From this point, the player, as Tommy, must engage in various jobs to find out who it was that set him up. The trail eventually leads to Ricardo Diaz, and after Tommy kills him, he takes over Diaz's mansion and uses his new-found resources to start his own criminal empire. The endgame sees Tommy engaged in a war against the Forellis, and in the end, he manages to secure his place by killing Sonny Forelli, the man that had sent him to Vice City in the first place.
The protagonist of the God of War series, Kratos is originally introduced as a ruthless general of Sparta that pledges his life to Ares in exchange for the power to crush his enemies, and Ares grants Kratos this boon. With the power he has been given, Kratos unquestioningly follows Ares's commands, destroying village after village until he one day realizes that among the dead in his latest conquest are his own wife and daughter. The ashes of his family are then bonded to his skin, giving him a ghastly white palor. Kratos vows revenge against Ares for what the god of war had led him to do. His further adventures see him fight against the rest of the gods of Olympus after he feels that he has been betrayed yet again.
In his lust for vengeance, Kratos is single-minded in the execution of violence. He kills almost all that stand in his way, god or mortal, even if they pose no particular threat to his goals.
First introduced as an antagonist in the Mega Man Legends series, Tron Bonne is a hotheaded, mechanically gifted member of a family of pirates that look for their fortune by stealing it. However, in The Misadventures of Tron Bonne, she stars as the protagonist, and must work to repay a loan shark debt incurred by her family in order to rescue her brothers, Teisel and Bon.