The Unarmed Marine is a hidden Easter egg on the level 343 Guilty Spark. His character model is that of Private Mendoza. It appears that Bungie wanted his model to load before the cutscene that he later appears in; they thus coded him to load early in the level, and simply used the area outside of the level as a storage area. He can be shot and killed, just like any other Marine.
To find him, you must stay inside the Pelican that drops you off at the beginning of the level; to do this, throw a plasma grenade as soon as you are able to. Stay inside the dropship as it flies off. After it travels a certain distance, the Pelican will drop you off outside of the level barriers. By exploring around, you will eventually come across the Marine.
Sometimes, when you're inside the structure, bodies will spawn all around where you came in from. Sometimes (if not always), if you surface just before the Jenkins cutscene, bodies will spawn all around and the Unarmed Marine's body will be where you fought the first Covenant on the level. If somehow you manage to get to the higher level after doing the trick above, Flood Combat Form corpses will be around the spot where the Unarmed Marine is standing.
In the Halo Combat Evolved Anniversary version of the level, a tree has been placed right in front of the Hidden Marine, making him harder to locate in the new graphics engine. If he is located using the old graphics engine, and then switched over to the new engine, he will be found partially phased inside a tree.