Ada Wong is the pseudonym of an otherwise-unnamed red-clad, secret agent of Chinese heritage.

Initially a private agent, Ada was drawn into the intrigue and treachery that is the corporate world when she was recruited by a rival compagny to participate in their activities against their primary rival, Umbrella, and later moved to another.

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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, 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. 
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She was sent to infiltrate the Arklay Laboratory, hidden beneath a mansion in the Arklay Mountains and was romantically involved with the new head researcher, John Clemens, as a means to steal intelligence for her company. John was very serious about his relationship with Ada, and even used her 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.

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At some point, she worked for Derek C. Simmons, who became infatuated with her. After his decision to destroy Raccoon City as a result of it's T-Virus outbreak, she left him and the job. Simmons, who had already gained strong feelings for Ada, eventually used Carla Radames in an experiment to clone Ada for himself. 



             Source : http://residentevil.wikia.com/Ada_Wong