Master sergeant
From Halopedia, the Halo wiki
In the Marine Corps, Master Sergeants may 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.
They are above Gunnery Sergeants and below Second Lieutenants. Their duty is to aid Colonels.
All Master Sergeants are senior Non-commissioned officers.
