In Greek mythology, he's:
The Greek god of war and unrestrained bloodshed, Ares was the big, violent dudebro of Olympus. This frequently brought him into conflict with other gods (especially Athena and the blacksmith god Hephaestus, whose wife, Aphrodite, Ares was constantly sleeping with), but generally he was just out to have some good, violent fun.
But in God of War, he’s:
Basically Satan, in that he strikes a deal with Kratos, saving the Spartan’s life in exchange for his soul. He then proceeds to use the deal to take away everything Kratos holds dear, and follows that up by challenging Zeus himself. So with the help of all the other gods, Kratos gets really huge and kills him.