Umm who is this guy? Oh thats The Master Chief! The Master Chief's Real name is John, as found out in "The Flood", the book. Master Chief is a secret project in which Master Chief is going to be the leader of a super-soldier army, its the only hope for survival. Master Cheif relies most of the time on Cortana.
This is Captain Jacob Keyes, who made the whole idea about Master Chief and Cortana (below). Captain Keyes has been working with Master Chief ever since he was a little boy. Keyes also made a program to make Cortana, who is a big help for Master Chief.
This is Cortana, she is a hot artificial intelligent robot (just kidding about the being hot part). She was originally for the Pillar Of Autumn, but she moved to the MJOLNIR suit of the Master Chief. Hey, she is sort of sexy now that I look at her.................
This is a Brute. It is new for Halo 2, and not much information has been givin about the Brute.
This is an Elite. It is one of the most powerful aliens in the game. It will make its apperance for Halo 2. 
This is a prophet, they are new for Halo 2 and they are said to be the leader of the covenant group.
This is a grunt. There are different types of Grunts throughout the game, and the Grunt will make its apperance in Halo 2. 
