From Halopedia, the Halo wiki
Private Parsons is a Marine with the 405th out of Diego Garcia. He appears in the Halo 2 E3 Demo but is not named in the final game. In the demo, after Corporal Perez shows John-117 around the building, Perez will come up to a Marine pinned down and ask, "Hey, Parsons! Is it clear?" A nearby explosion will startle and shake the two Marines, as Parsons responds, "You tell me!" before Perez provides covering fire for him.
List of appearances