Dr. Callos was the head of research for a secret Umbrella Corporation lab located outside of the Nevada town of Saguaro Wells. Dr. Callos' goal was to genetically combine bats with humans so as to be able to spread the G-Virus among the greatest number of people in the shortest amount of time. An extremely strict and unforgiving leader, Callos was known for punishing his subordinates with death at the slightest provocation, and thus his support staff lived in fear of him.
He and his team succeeded in creating a number of human-sized bat creatures, the Night Stalkers, which they controlled using an advanced sonar array. They would send the bats out into Saguaro Wells each night to abduct the townspeople, and bring them back to the lab to turn them into bat monsters, too. His plans were thwarted by the efforts of former RPD officer Leon S. Kennedy, who allowed himself to be captured so he could get close to the sonar array and destroy it. With nothing to control them, the bat creatures turned on Callos and his colleagues. Callos was killed and the base was then destroyed by a US Air Force strike team.