" So you were probably wondering why we didn't want to commit to two player co-op online over Xbox Live. We certainly got plenty of mail asking, no, demanding that we make it happen. Of course we were working on it, but we were also working on something better. Not two player co-op. Not three-player co-op, but up to four player co-op. Online. On Xbox Live, or sure, System Link if you prefer", said Bungie on their weekly update.
The post goes on: " Acting as a team is a fantastic new twist to the Campaign gameplay - something that works as well on foot, in close confines as it does on the battlefield, with large scale vehicle mayhem. Scared of Jackal snipers? Send out a scout to see what dangers lie ahead. Terrified of an open field? Flank your enemies and swipe their rides. The combinations and scenarios are endless - and you'll be able to enjoy them time and time again in Saved Films of your co-op exploits."
Instead of Halo's slightly surreal compromise of two identical Master Chiefs, Halo 3 opted for 4 different co-op characters. Player one will control the Master Chief himself, player two will control the Arbiter and players three and four will get the chance to control two brand new Elite characters: N'tho 'Sraom and Usze 'Taham. All 4 characters will have identical abilities, with the only differences being their appearance and starting weapon.