Rear admiral (lower half)
From Halopedia, the Halo wiki
Rear Admiral (Lower Half) is a senior commissioned officer rank of the UNSC Navy.
This rank is above Captain and below Rear Admiral (Upper)
It is equal with the rank of Brigadier General in the UNSC Marine Corps.
In informal settings, Rear Admirals (Lower Half) are typically referred to simply as "Admiral".
Known Rear Admirals (Lower Half)
See Rear Admiral.