The odst's (a.k.a Helljumpers)

The Marines on the Pillar of Autumn are a rugged and diverse assortment of soldiers, fighting a losing battle against the Covenant's superior weaponry and numbers. Under the leadership of Captain Keyes they wage a furious struggle against the Covenant. They're the best of the best. But they're only human.

It is rumored that in the second Halo you will have many more Marines fighting with you. It is also said that you will be able to trade weapons with them and communicate more.