Throughout the Resident Evil series, several Umbrella-owned paramilitary units were introduced, driving the plot and providing new enemies and allies.
Umbrella Security Service (U.S.S.)
Other games give further background for the Umbrella Special Forces. According to Code: Veronica, USFU agents are trained at the Rockfort Island facility.
One of the more notable members of the USFU, HUNK, appears first in a minigame in Resident Evil 2, where he is given the mission of retrieving the G-virus from the research facility under Raccoon City. Canonically, he is the only survivor of his squad's mission.
USFU agents are always depicted in solid-color grey fatigues, wearing protective vests and gas masks (except in Resident Evil: Apocalypse, where they are depicted in solid black fatigues, vests, and a type of enclosed helmet). They report directly to the highest ranking officers in Umbrella. Unlike the UBCS which performs more visible missions such as rescuing civilians, this unit maintains total secrecy and a small size. Also unlike the UBCS, this unit's personnel are not considered to be expendable. Members of the USFU are ex-military, highly trained, and dedicated to protecting Umbrella's various assets.
Umbrella Biohazard Countermeasure Service
The UBCS was founded by former Soviet Colonel Sergei Vladimir. The service is mainly composed of "nightmen", mostly former mercenaries and war criminals recruited by Umbrella and trained to deal with "problems caused by illegal products." In Resident Evil: Apocalypse and Extinction they are referred to as the Umbrella Biohazard Countermeasure Force.
Umbrella Trashsweepers unit