Ada Wong is a mysterious femme-fatale, who's motives are unknown when she is first name-dropped in Resident Evil, in one of the many documents found in the game. She is mentioned in a letter from a dying researcher to her. Ada Wong makes her first proper appearance in the special edition version of Resident Evil 2, where she supports one of the main player characters, Leon S Kennedy. Her actions throughout the Resident Evil 2 storyline are concluded in Resident Evil: The Umbrella Chronicles. She later appears in Resident Evil 4, again alongside Leon, during the Los Illuminados Incident.
In the Playstation 2 and Wii versions of Resident Evil 4, Ada Wong is a playable character in her own spin-off campaign, called Separate Ways, as well as a playable character in the Mercenaries minigame available in all versions of the game. In the most recent installment, Resident Evil 6, Ada Wong once again is a playable character in her solo campaign which is unlocked once the player complete the game.
Ada Wong is first mentioned when a researcher at the Spencer Mansion, John, leaves a letter for her telling her of the events that have transpired there and giving her his password (her name) should she arrive to try and find evidence to use against Umbrella. What John didn't know was that Ada was a spy for a rival corporation using him to gain access to Umbrella's secrets. John died first due to being infected by the T-Virus, and then from a gunshot wound to the head.
Ada arrives in Raccoon City under the pretense of looking for John. In reality she is there to steal any info on the G-Virus. She is present when the T-Virus outbreak occurs and manages to make it to the Raccoon City Police Station before it is barricaded. She is very adapt at defending herself and she takes a shot at rookie police officer Leon S. Kennedy. Leon is immediately smitten with the striking Asian-American woman but Ada generally just ignores him and his obvious advances. Ada is initially searching for Ben Bertolucci who she believes has information on John. Ben has locked himself inside a jail cell and refuses to talk or leave the jail cell. Ada leaves looking for a way to gain access to the sewers so that she can get to the underground lab.
While she is in the sewers she encounters Annette Birkin. Annette is weary of anyone in the sewers and tries to kill her. Leon saves Ada's life (taking a bullet for her). Ada bandages Leon's wounds and begins to warm up to the young officer (despite herself). She then leaves Leon to go after Annette. When Annette learns of Ada's identity she realizes that she was John's girlfriend and that she is also a spy for a rival corporation. She informs Ada of John's fate and tells her how the outbreak occurred. When Annette sees Sherry's locket she realizes the secret the woman possesses and the two women struggle and Ada is victorious (Annette falling over the rail into the sewer).
Ada is rescued by Leon from a mutated Alligator that has taken residence in the sewer. The duo then fight their way to the underground facility where Ada is wounded by William Birkin. She convinces Leon to leave her after which she leaves the safety of the office to try and find a G-Virus Sample (unknown to her, she has the vial located in Sherry's locket).
Ada finally returns the favor when Leon is fighting Mr. X (a T-103). She gets its attention and then manages to wound the Tyrant but not before sustaining massive internal injuries herself. As she appears to die she and Leon kiss. Leon leaves her but in reality she is not dead, just faking. Somehow Ada manages to locate a rocket launcher and gives it to Leon to defeat the combat mode of the T-103 that had injured her. Leon destroys the beast and Ada departs to go meet her contact.