Fire Team Charlie
From Halopedia, the Halo wiki
Fire Team Charlie was a United Nations Space Command Orbital Drop Shock Trooper unit stationed aboard the UNSC Pillar of Autumn that actively participated in the Battle of Installation 04.
During the raid on the Truth and Reconciliation, Cortana broadcasts: "Cortana to Fire Team Charlie. Move up on the left. You should be able to flank the enemy."
Connection with Foehammer
They seem to be in some connection with Pelican Dropship Echo 419's Pilot 'Foehammer.'. In the level Halo, Fire Team Charlie knows the pilot of dropship Echo 419 - 'Roger, Echo 419. This is Fire Team Charlie. We read you 5-by-5. That you, Foe Hammer?' .
Appearances
A different Fire Team Charlie was seen in Halo: Combat Evolved first in the first scene of level Pillar of Autumn, but were not named until the second level, Halo. This Fire Team Charlie was part of Staff Sergeant Johnson's platoon. The ODST Fire Team Charlie was seen in the level Truth and Reconciliation, led by Sergeant Parker. This leads to the speculation that UNSC Marine platoons and companies are divided into fire teams named by letters in the phonetic alphabet (Alpha, Bravo, Charlie, etc).
Speculation
Fire Team Charlie and Fire Team Zulu may be in the same company, maybe even the same platoon, although this is pure conjecture and not definitively or even implictly stated.
