Hidden soldier in Cairo Station
From Halopedia, the Halo wiki
At the Fire Control Station aboard Cairo Station, where the Covenant Bomb is, there is a secluded Marine. Right when you exit the elevator to the Fire Control Station, take a left. Then take another until you're at the very back of the room. There you will see a fellow UNSC soldier holding an SMG. Upon seeing you he will sometimes say, "Thank God you're here sir." Or several other phrases that indicate that he is very happy to see someone friendly. He might have even been in charge of protecting the Fire Control Center. You can keep pushing him, albeit hard, by walking into him.
Pushing your fellow Marines into battle also works on other levels, although they're less than eager to join you.