Master Chief

 

The last of the UNSC cyborgs stored abord the Pillar of Autumn in an escape from Reach, UNSC Headquarters.  He can wield basically any weapon and pilot and drive any vehicle.

 

Cortana

 

An AI construct that can tap into radio signals and also has a lot Forerunner data.

 

Keyes

 

Your captain and leader of the troops.

 

Sarge

 

Commander of the Marines and other UNSC forces.

 

Marines