Private
From Halopedia, the Halo wiki
Private is the lowest enlisted rank of the UNSC Marine Corps and UNSC Army, and is equal in rank to Crewman Recruit in the UNSC Navy. For most recruits into the Marine Corps and Army, when they complete their basic training, they become a Private, and are further assigned to a specific unit or platoon. The next level up from Private is Private First Class, or PFC.