10. Master Chief & The Arbiter (Halo 2)
Every great hero needs a great adversary, and in the first Halo: Combat Evolved, no enemy gave the Master Chief a run for his money like the Covenant Elites. But in Halo 2, the writers came up with a twist: not only would players be introduced to the Arbiter — a dishonored Elite sent on a suicide mission by his commanders — but would get to play as him as well. Splitting the story between the Chief and the Arbiter meant players could wield an Energy Sword and command Elite troops, but when the human and alien heroes joined forces on the battlefield against a common enemy (with or without a co-op partner), the villains of the series didn’t stand a chance.