From Halopedia, the Halo wiki
UNSC Marine Corps
Brimley is a UNSC Marine and an Orbital Drop Shock Trooper in the 105th ODST Division. He served during Operation: FIRESIDE in 2552, and was a good friend of Langston.
Before the deployment to Ariel, Langston's death at the hands of John-117 in 2525 led him and Romeo into an argument on the Spartans' fighting prowess, and eventually into a full-on fight, in which he overpowered Romeo before Dutch intervened.