Paranoid Marine

This UNSC Marine was presumably a member of Fire Team Charlie, deployed to what was thought to be a Covenant weapons cache. In reality the fire team was headed for a Flood Containment Facility.

He was inserted into the swamp by Pelican Victor 933, along with Sergeant Johnson, Private Bisenti, Private Mendoza, and Private Jenkins. The squad met up with Captain Keyes, along with Privates Kappus and Riley, after discovering the remains of a dead Elite. After entering a large hall, the squad was ambushed and attacked by small, squidlike creatures. This Marine witnessed the death of almost his entire unit, and fled the battle.

Long after the Flood had finished their massacre of Marines and Covenant alike, Spartan-117 came across this Marine. The soldier was a wreck, firing at anything that moved with his pistol, and hollering about "monsters." In Halo: The Flood, it is said that the Master Chief told him to leave the building, but the Marine ignored him, and carried on babbling. It was once unknown what happened to the Marine afterward but further inspection revealed his dead body near a pile of rubble.

Trivia

 * It is possible to see his dead body after the Flood outbreak. To do so you must get to the part were the Flood attack without killing the marine.  Backtrack to where he was and you'll find his body in a pile of rubble surrounded by blood. It is unknown if he killed himself or if he was killed by the Flood and was not infected.
 * A homage to the Halo: CE, the Halo 3 Crazed Marine is voiced by the same person, and features similar mannerisms to the original. Found in Floodgate (Level), in the Flood-ravaged Command Center, he is surrounded by the corpses of his former platoon, killed to stop them from becoming infected by the Flood. When the Master Chief and the Arbiter arrived, he was huddled in a corner, babbling in his despair, and how the Flood "talked" to him, constantly pointing his pistol at his head. Whether this marine is intended to be the same character or his counterpart from the first game is unknown, though improbable.