Master sergeant

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

Master Sergeants may also be referred to by the nickname of "Top". This usage is an informal one, however, and would not be used in an official or formal setting. Use of this nickname by Marines of subordinate rank is at the rank holder's discretion.

First Sergeant between Master Sergeant
In the United States Marine Corps, 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 UNSC Gunnery Sergeants is unconfirmed.

Responsibilities
When a First Sergeant is not available, a Master Sergeant may take up the role as the senior NCO for company level units. They then become the assistants and advisers for the company commander. This is seen during the events of Halo: The Flood, when Master Sergeant Lister became First Lieutenant Melissa McKay's senior NCO.