Ada Wong is a red-clad, cunning and formidable secret agent of Chinese heritage with a mysterious past. Initially a private agent, Ada was drawn into the intrigue and treachery that is the corporate world when she was recruited by The Agency to participate in their activities against their primary rival, the Umbrella Corporation. Ada remained an operative of the organization, even as the battle shifted from the soon defunct Umbrella to the mysterious "S" Corporation, with Ada posing as a double-agent inside the corporation on the Agency's behalf, pretending to work with Albert Wesker and monitor his activities to gain power. Though she was affiliated with unscrupulous and questionable characters and organizations, Ada was shown to have integrity and some measure of mercy and kindness, leaving her true affiliation unknown.
Not much is known about Ada Wong's past or motives. What is known is that Ada was a corporate spy working for an unnamed competitor of Umbrella, often referred to as "The Organization" or "The Agency", that intended to procure all sorts of biological weapons and specimens. She was also a trained assassin, who was proficient with several types of weapons and fighting techniques. Presumably, Ada joined the Organization for personal reasons outside of her company's interests, though she has yet to reveal what they are.
She was sent to infiltrate the Arklay Research Facility, hidden within the Spencer estate in Raccoon City and was romantically involved with the new head researcher, John, as a means to steal intelligence for her company. John was very serious about his relationship with Ada and even used her name as his computer account's password. John lost his life, due to the T-virus outbreak caused by James Marcus, after writing one final letter to Ada instructing her to get evidence of Umbrella's experiments and expose them to the public. She never read the letter because she was not in Raccoon City at the time.
Several months later, Ada was given another mission by her employers. Like Umbrella special agent HUNK, her objective was to steal a sample of the G-virus from the self-injected and mutated William Birkin in Raccoon City during the T-virus outbreak.
Ada went to the Raccoon Police Department to find a reporter named Ben Bertolucci, who claimed to have vital information about the Umbrella Corporation. In her search of the department, she met with surviving officer Leon S. Kennedy. With Leon's help, the two were able to find Ben, who locked himself in the precinct's prison. Ben provided the two with documents detailing about the conspiracy between Brian Irons and William Birkin, the designer of the G-virus, but not before he was killed by a mutated Birkin.
Ada and Leon made their way into the sewers as they attempted to flee the infested city. The two cooperated with each other and Ada even treated Leon's wound after taking a bullet for her, fired by Annette Birkin. While silhouetted, Ada also threw Leon a rocket launcher to dispatch the Tyrant T-103 which had been stalking them for sometime. They found a lift which led to Umbrella's Underground Lab, but before they could reach the lab, Ada was struck by a rapidly-mutating William Birkin and was critically injured. During the ensuing confusion, Ada was seriously injured protecting Leon and presumed dead (see:Canon in Resident Evil).
She managed to get out of the laboratory and made her way to the Apple Inn where she was due to meet with an Umbrella worker. When she got to the room, she found that the man had already committed suicide. Even worse, Wesker was watching her live on a computer. When Ada shows him the G-virus sample she recovered from William Birkin, he told her there were moments left until the city was wiped out by a missile. He helped her by telling her a helicopter sent to evacuate the last of Umbrella's workers, who were revealed to be Sergei Vladimir and Ozwell E. Spencer was set to leave the city shortly.
Providing Ada with a Hookshot, Wesker expressed his interest that she survived with the G-virus sample. She fought her way past the hoard of Zombies and defeated a large Tyrant monster on one of the highway bridges leading outside the city. She then used her Hookshot to grapple onto the Umbrella helicopter as it passed overhead. While she ran as the copter flew away, a Hunter chased her and jumped to grab her as she was lifted into the air. The Hunter failed to grab Ada by mere inches, and instead got her high heel.
Six years after the Raccoon City incident, Ada was sent to an undisclosed village in Europe that had been taken over by Osmund Saddler, leader of the "los Illuminados" religious cult, after he infected the villagers with Las Plagas. Her mission was to steal a specialized sample of the Plaga held by Saddler. During her mission, she met once again with Leon, who was on a mission to save the president's daughter, Ashley Graham, from the same cult. Wesker sent another loyal agent of his, Jack Krauser, who thought highly of Wesker's plan to remake Umbrella and the world under his leadership, to work alongside Ada in this mission. However, Ada and Krauser had difficulty seeing eye to eye, with Krauser distrusting Ada and believing her to be a threat to Wesker. Despite her pointing out that she had worked with Wesker longer than he did, he still kept his eye on her and promised to kill her should she try anything clever. Ada was upset by Wesker's desire to dispatch Leon and was reluctant to recieve his order to kill him herself. Later, she was even more angered when Wesker decided to leave Leon to Krauser. She then decided to not complete her task exactly as Wesker wanted and would come to save Leon when Krauser came close to killing him.
Ada was responsible for a number of things Leon encountered during his mission, such as puzzles and traps set off in a certain way and so forth. Although her mission was to retrieve a master sample of the Plaga, Ada's primary objective was to get in good favour with the mysterious organization she worked for and was also conspiring against Wesker. Arguably, if it wasn't for Ada's role behind the scenes, Leon's mission would have been a failure. She also aided Leon in battle and even saved him from the influence of the Plaga during the mission. Another of Ada's accomplishments was the destruction of a previously unseen battleship that Los Illuminados had at their disposal.
Shortly before the climax of Ada's mission, she is attacked by what's left of Krauser, freshly burned and scarred from his battle with Leon. Ada takes it upon herself to go on the offensive and kill Krauser, who by now is almost completely corrupted by the Plaga. Having studied his style and mutation closely before, Ada is more than a match for Krauser and he is killed, presumably for good. Shortly after this, Ada covers Leon and Ashley's escape from Saddler by blocking the passage, trapping both Ada and Saddler in the same room. A battle occurs, where Ada fights Saddler's human form. With incredible strength, Saddler is quite a challenge, but Ada is eventually able to overwhelm him through her skill. However, this isn't the case, as Saddler comes to his senses shortly after and gets the drop on Ada. Using Ada as bait to trap Leon, Saddler is shocked to realize Leon's Plaga has been destroyed as Leon frees Ada from captivity. Ada and Leon fight Saddler during the final battle and as Leon confronts the monstrous cult leader, Ada struggles in a race against time to find Leon a powerful weapon to use against him. Rushing through hordes of los Ganados, Ada finds a specialized rocket launcher and throws it to Leon at the critical moment of the battle.
Eventually, Ada is able to steal a master Plaga sample from Saddler's island base after the battle. After she obtains the sample, she gives Leon and Ashley a means of escape and makes her own on an Organization helicopter. As she rides off, Ada is comforted by the warm light of the sunrise on the horizon. However, she can't help but think that while this mission is over, the battle has just begun.
Ada had obtained the sample, the goal of her mission and one step of Wesker's plan to resurrect the Umbrella Corporation. However, the two became suspicious of each other. Under orders of the Organization, Ada double-crossed Wesker and delivered a fake version of the sample to him. Ada knew, however, that Wesker isn't stupid and that tough times lay in wait.