Master chief petty officer
From Halopedia, the Halo wiki
Master Chief Petty Officer is the ninth, and highest enlisted rank in the UNSC Navy, just above Senior Chief Petty Officer and below Ensign, and they are the equivaent to Master Gunnery Sergeant in the Marines, and is a Non-commissioned officer. They are referred to as Master Chief in most circumstances.
File:150px-Cheif Again.jpg
only known person in halo to hold the rank
