The U.B.C.S. were first introduced in Resident Evil 3 as Umbrella's own privately funded army. They were sent in to Raccoon City to save civilians, completely unaware that it was due to the T-Virus concocted by Umbrella that was the cause for the whole outbreak to begin with. The unit is primarily made up of mercenaries and war-criminals, with practically no ties to the company itself. As such, Umbrella see them as expendable and are more than willing to throw them to their creations under the pretence of helping civilians, but with the true agenda of collecting battle-data.
They as such differ from Umbrella's U.S.S. unit, which are more closely related to the company and are therefore suited for more covert operations, some of which may even clash against the U.B.C.S.
Appearances in Resident Evil
The most well known three within the group are Carlos Oliveira, Mikhail Victor and Nicholai Ginovaef. Carlos played a supporting role through Resident Evil 3 as a constant ally to Jill Valentine during the final few days before Raccoon City's destruction. Mikhail only played a minor role and was mostly seen lying on a tram seat because of severe battle-wounds. Nicholai, on the other hand, played the part of the human antagonist for the game. He was one of the ''Supervisors'' who were given a more detailed briefing of the operation, and were there to gather combat data as the U.B.C.S. battled the B.O.W.s.
Carlos would always survive the Raccoon City ordeal, as would Mikhail also always be killed by the Nemesis while trying to protect Jill. Nicholai's fate, however, is ambiguous depending on the ending. He could either be killed by Nemesis, killed after being shot down in his helicopter by Jill Valentine, or will manage to escape Raccoon City in the aforementioned helicopter if Jill chooses not to combat him.
Though the U.B.C.S. haven't been encountered any further than the Raccoon City operation, they would still be found again in the Outbreak titles which are based around the said operation. Though Carlos and Mikhail aren't seen nor even mentioned, Nicholai makes a cameo in Outbreak File 1 where he blows up the Raccoon City University and assassinates rogue Umbrella scientist, Greg Mueller. Multiple nameless U.B.C.S. soldiers are seen throughout as well, mostly while combating Greg's own creation, Thanatos.
They also appear during Outbreak File 2 as well, once again acting as B.O.W. fodder. Though another named U.B.C.S. soldier is introduced as Arnold. He is a selfish and potentially sociopathic marksman who's more interested with simply killing zombies than trying to save civilians.