A woman of Chinese descent, Ada is a cunning and formidable secret agent for the mysterious and powerful group known as "Organization". Ada is first mentioned in the original Resident Evil, in a letter written by a dying Umbrella Corporation researcher named John, and makes her first appearance in Resident Evil 2 (revisited in Resident Evil: The Darkside Chronicles), secretly working as a spy for an unnamed company, sent to recover a sample of the deadly G-virus from the Umbrella's lab in Raccoon City. Ada poses as a bystander who is searching for her missing boyfriend, John, when she meets Leon S. Kennedy, a rookie police officer trapped in the city. Her secret agenda is exposed, but (depending on which scenario is played) she is seriously wounded by either Annette Birkin or the Tyrant T-103 and falls from a massive shaft. An epilogue in Resident Evil 3 as well as a fictional documentary in Code: Veronica confirm that Ada survived the ordeal. Ada's role in Resident Evil 2 is also explored in Resident Evil: The Umbrella Chronicles and Resident Evil: The Darkside Chronicles.
Her next appearance was in Resident Evil 4, where she assists Leon in his mission to rescue the new U.S. President's daughter Ashley Graham, although her true objective is to obtain a sample of the Las Plagas parasites for Albert Wesker. Ada appears in the game's sub-scenario "Assignment: Ada", as well as in the "Mercenaries" mode. A new scenario starring Ada, titled "Separate Ways", was added to the PlayStation 2, PC and Wii ports, which depicts the events of the main game from Ada's perspective, as well as a fictional documentary titled "Ada's Report", where she describes her involvement with other characters in the story.
Source: Wikipedia, "Ada wong", available under the CC-BY-SA License.