"Prepare to fight a God!" Ares to the Spartan before fighting him.
Ares: The God of War and The Spartan's protector and guide. Ares is capable of creating mirror images which can deal damage and raise the dead back alive. He is ultimately killed by the Spartan.
Ares is ever present during the game, revealing himself to the Spartan (as a voice in his head) and offering advice/missions to the Spartan. Ares will also chip in demanding the Spartan 'kill and kill again'. At the end of the game it is revealed, through both Tiberius and Ares, that the latter had orchestrated much of the events prior to the final fight.
The reason for Ares's betrayal of the Spartan is one of illicit love. Historically, it is suggested that Ares had a long affair with Aphrodite, while she was married to Hephaestus. In the game, one of Aphrodite's handmaidens reveals the affair to Hephaestus.