From Halopedia, the Halo wiki
Marcus is a variant of the UNSC Marine face bitmap found in Halo: Combat Evolved, modeled after the game's art director Marcus Lehto. It is used for Sergeant Stacker's appearance. One can find it by opening a Halo: Combat Evolved map in Halo Map Tools. Not including Avery Johnson, there are six variants of face bitmaps. They are in DDS DXT1 texture format.