If you mean in campaign mode you play as that guy (he is called the arbiter) further on in the game. From about level four (I think) onwards you play as him about half the time. If you mean in multiplayer you go to settings then edit your profile and change your character from a Spartan to an Elite. This is not exactly the same guy but is something like him.
I don't pretend to be an expert, so sometimes I get it wrong.