Sony's announced that players will now be able to play through God of War III as Kratos' brother, Deimos.
The only way to unlock Deimos is through a code supplied by the game God of War: Ghost of Sparta on the PSP.
Deimos will simply replace Kratos' skin on his character model but will also come with a few upgrades. He's more powerful and has four times as much health, magic, and gold orbs than Kratos does. His combat damage is also increased quite a bit. So in short, he's Kratos, but better!
Deimos will be available in Europe some time in November. This remains unconfirmed but in any case, it will definitely be out this year.