First sergeant

First Sergeant is an E-8 rank of the UNSC Marine Corps and 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. It is a staff non-commissioned officer in the Marine Corps and a non-commissioned officer in the Army. In the UNSC Air Force, First Sergeant is not a rank but it is a duty indicated by a diamond in the center field of the insignia of a Master Sergeant, Senior Master Sergeant, or Chief Master Sergeant.

First Sergeant and Master Sergeant
For the United States Marine Corps, in their annual performance evaluations, eligible Gunnery Sergeants 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.

Whether this carries over into the UNSC Marine Corps is unconfirmed.

Responsibilities
Assuming the rank still exists, First Sergeants function as the senior NCOs for company or squadron level units. They advise and assist the company or squadron commander directly, and are responsible for all other enlisted soldiers, marines, or airmen in the unit. Although technically the same rank, they are given more authority than Master Sergeants.