First sergeant

N.B.: This rank has not yet been mentioned in the Halo Universe.

First Sergeant is the eighth enlisted rank of the UNSC Marine Corps and ninth enlisted rank of the UNSC Army, and equal in pay grade to Master Sergeant, although the two have different responsibilities. First Sergeant is equal to the rank of Senior Chief Petty Officer in the UNSC Navy.

First Sergeant vs. Master Sergeant
For the UNSC Marine Corps, in their annual performance evaluations, eligible Gunnery Sergeant's indicate whether they wish to be considered for promotion to Master Sergeant or First Sergeant, and thus whether they enter the promotional track for Master Gunnery Sergeant or Sergeant Major. Once promoted, the promotional track is permanent; lateral movement between the two programs is not possible.

One of the major differences between the two ranks is that First Sergeants have a general command status, while a Master Sergeant's role is as a provider of technical military leadership.