He is found guarding the Stadium restrooms, and is armed with an M16A2assault rifle. In dialogue he tells players to not loiter near the bathrooms and to do his/her business before promptly returning to the field. He says this to eliminate the risk of survivors forming groups and planning escapes. Nothing else is known about him.
Strangely, even though he is an Army sergeant, his name is Private Ramirez. This is most likely an error by the developers.