Ada's past life before 1998 is a complete mystery. Her ethnicity and nationality are left unconfirmed, and when and where she was born are also unknown. Even her birth name is not believed to be real.By 1998, 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.

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 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.

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 its 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.