Rebecca makes her first appearance in the original Resident Evil, as a supporting character who assists Chris Redfield as a fellow officer in the fictitious special police force STARS in the United States. She arrives at the Arklay Research Facility before the STARS Alpha Team, after her unit is scattered across the Arklay Mountains. In the game, Rebecca is an 18-year-old member of STARS Bravo Team, being in charge of rear security and also serving as the team's field medic. Her introduction and other appearances throughout the game vary depending on the path the player chooses; she can be controlled by the player during certain portions of the story.
In Resident Evil Zero, which serves as a prequel to Resident Evil, Rebecca Chambers is one of the two main characters. In this game, Rebecca comes across the Ecliptic Express train while investigating a series of murders within the Arklay Mountains. She encounters an escaped convict, Billy Coen, who helps her. The two discover that the attacks have been masterminded by James Marcus, formerly one of the Umbrella Corporation's top scientists, who has reanimated himself to kill his murderers.
Rebecca also appears in The Umbrella Chronicles, which summarizes the events of Resident Evil Zero and details her experiences between Resident Evil Zero and Resident Evil; the game pairs her with fellow STARS Bravo Team member Richard Aiken, as they work together to fend off hordes of undead creatures.
She is also a playable character in Resident Evil 5 in its Mercenaries Reunion mode of the game's Gold Edition and returns in Resident Evil: The Mercenaries 3D.
Source: Wikipedia, "Rebecca Chambers (character)", available under the CC-BY-SA License.