Carlos Oliveria was a corporal in the UBCS Delta Platoon A Squad, among the many sent into Raccoon City by Umbrella, ostensibly to rescue civilians. Extremely cheerful and optimistic, perhaps even naive, Carlos honestly believed that the objective of the UBCS was to save the innocent citizens of Raccoon, little knowing that the Umbrella higher-ups really just wanted him to die so that a few select "supervisors," including his sergeant Nicholai Ginovaef, could collect the combat data of Umbrella's BOWs. He continued to believe this even after the zombies had whittled his unit down to just himself, Nicholai, and a badly-wounded Mikhail Victor.
After making a call for help over the radio, Carlos became seperated from Nicholai and Mikhail and encountered Jill Valentine in the area near the Raccoon Press offices. Not knowing that she was a former STARS member and thus an enemy of Umbrella, Carlos was unsure at first why Jill seemed hostile towards him once he revealed that he was with the UBCS. Regardless, he invited her to join him and his two remaining teammates in their quest to get to the designated landing zone for the UBCS extraction helicopter at the St. Michael Church. In the middle of being pursued by Nemesis, Jill had little choice but to agree.
Eventually Carlos came to realize that his own company had betrayed him when he learned that he and his men had been nothing but expendable zombie bait, a fact learned when he found Nicholai trying to kill another surviving teammate, Tyrell Patrick (Ken Franklin in the novels) at the Raccoon Hospital while he was there trying to find medicine for Jill who'd been badly wounded in a fight with Nemesis. The extremely disillusioned Carlos was in denial at first, but then came to accept the fact that both Nicholai and Umbrella, who he'd thought were his friends, were in fact his enemies.
Thus, he actively aided Jill in her campaign to escape the doomed Raccoon City and destroy Nemesis. He escaped with her thanks to the help of her fellow ex-STARS member Barry Burton, who flew them safely out of town in his helicopter before a nuclear bomb hit and destroyed Raccoon.