Bloody UNSC Marine
From Halopedia, the Halo wiki
The Bloody UNSC Soldier is an Easter Egg found in the Halo 2 level Cairo Station. When you reach the loading hangar where you must defeat the first Covenant boarding party, make sure that you save the lives of all the Marines with you. When every last Covenant is dead, look at your accompanying Marines' faces. One in this room will be cut, bruised, and covered in blood. This is the only place in Halo 2 where such a damaged Marine can be found. You can also see a Bloody UNSC Marine in the Art of Halo on page 14.
Occasionally, you can see a bloody Marine on the Easy and Heroic difficulty setting, too.
These Marines seem to have less health than the other Marines, but this has not been confirmed. On any level above Normal, they seem to be one of the first to die. One is voiced by John Kassir. In addition, they are also one of the few Marines that never wear a cover (headgear) during the level (most other covers are the Dress hat, Sergeant Hat, or no hat). His overall accuracy (including grenades and rifle fire) is the same as a normal Marine. He also appears to have no pouches on his breastplate. Bloody UNSC Marines, may have been at one time, going to be used as killed Marines. For example, in Halo 3, when a Marine is killed, their body turns bloody.
