Ada Wong appears to be just another supporting character that you encounter through Leon's campaign. She's supposedly there looking for her boyfriend, John. Of course, this isn't actually the case, and Ada is actually a spy from one of Umbrella's rivals, there to steal a sample of the G-virus.
She serves as Leon's partner for a good portion of his story, but she eventually chooses to sacrifice herself to allow Leon the chance to escape from Mr. X. She doesn't actually die, though, and goes on to appear in future games, but that doesn't make the moment less impactful.