Not much is known about Ada Wong's past or motives. What is known is that Ada is a corporate spy working for an unnamed competitor of Umbrella, often referred to as "The Organization" or "The Agency", that intends to procure all sorts of biological weapons and specimens. She is also a trained assassin, who is 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 Mansion 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 turning into a zombie following the Mansion Incident, after writing one final letter to Ada instructing her to get evidence of Umbrella's experiments and expose them to the public - evidently believing in their relationship right up to his death. In the letter, Ada is also referred to as not having contracted the T-virus during the incident. How and when she escaped the Spencer Mansion is still unknown.
Several months later, Ada was given another mission by her employer. This time, her objective was to steal a sample of the G-Virus from 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 either by a chest-bursting parasite or by the 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. They find a lift which leads to Umbrella's Underground Lab, but before they could reach the lab, Ada is struck by a rapidly-mutating William Birkin and is critically injured. During the ensuing confusion, after Leon confronts Annette for the last time, Ada is seriously injured and presumed dead (depending on the scenario the player is playing, she either falls to her death from a catwalk after being shot by Annette or dies after being slammed by the Tyrant). Despite her death, Wesker (who claims he still had use for her) saved her behind the scenes. Ada managed to recover and provided the rocket launcher that Leon or Claire used to defeat the Tyrant. Although Ada and Wesker were unable to retrieve the G-Virus sample (having lost it to HUNK), they managed to find William Birkin's corpse and use it as a specimen.
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. Ada is responsible for a number of things Leon encounters during his mission, such as puzzles and traps set off in a certain way and so forth. Although her mission is to retrieve a master sample of the Plaga, Ada's primary objective is to get in good favor with the mysterious organization she works for and is 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 aids Leon in battle and even saves him from the influence of the Plaga during the mission (ironically, taking Leon's earlier advice about using a knife in close combat). Another of Ada's accomplishments is the destruction of a previously unseen battleship that the Los Iluminados had at their disposal.Shortly before the climax of Ada's mission, she is attacked by what's left of Jack 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 Ganado, Ada finds a specialized rocket launcher and throws it to Leon at the critical moment of the battle.Eventually, Ada is able to steal the 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, Wesker has become suspicious of Ada's loyalities, due to her secret connections with another major pharmaceutical corporation, 'S'. Under orders of the Organization, Ada double-crossed Wesker and delivered a fake version of the sample to him. Ada knows, however, that Wesker isn't stupid and that tough times lay in wait.
Or at least intell from "S" believed so. Ada in fact had remained loyal to Wesker, bringing him the real plaga parasite sample, having switched the fake with the original. Ada's motives for helping Wesker are still unknown, though some speculate that she in some way cares for Wesker, although she does in fact at the same time resent him, making it a bit complicated for her. To make matters worse, the same thing could be said about Leon.
During the Raccoon City incident, Ada admits to loving Leon before she dies, and it seems that her love may be genuine as she seems to truly care for him during the Los Plagas incident (which is especially hinted at in 'Separate Ways' where she is genuinely shocked and worried when she sees Leon being carried by a Ganado. Of note is Wesker's comments in his report that in Resident Evil 2: "her love for him drove her to her death." ) Though in fact she did not die during the Raccoon City incident, she in fact was saved by Wesker, and revived. Some speculate that he injected her with some of the Virus, not enough to cause mutation, just enough to revive her, which shows the tiniest hint of concern for Ada from Wesker.
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