UBCS:Umbrella Bio-hazard Countermeasure Service
The first time you truly meet any UBCS member (if memory serves right) is in Resident Evil 3:Nemesis
The more popular characters are Carlos Oliveria, Nicholai Ginovaef, & Mikhail somethin or other. UBCS is a merc force of Umbrella 'designed' for 'cleaning up their mistakes'
RE:3 Merc's mode allows you to play as either Carlos (medium) Mikhail (easy) or Nicholai (Hard)from the UBCS. I base them as easy or hard depending on what they originally start out with, i.e. Nicholai starts w/ a handgun, a knife, and an herb or two.