One of the primary antagonists in Resident Evil 6, Carla is initially portrayed as an uncharacteristically cruel Ada Wong. However, it is eventually revealed in Ada Wong's own campaign that she is in fact a clone Derek C. Simmons created using Carla Radames (without her consent), Ada Wong's DNA, and the C-Virus.
*Taken from Resident Evil 6's own in-game file on Carla Radames*
Carla was responsible for the discovery of the C-Virus, and a close confidant of Derek C. Simmons. A prodigy in her youth, Carla completed a doctorate course in genetics when she was only fifteen, after which she was recruited by Derek to work for him manufacturing viruses and B.O.W.s. Derek praised her for her intelligence and successful research, and his praise increased her devotion to him. One way she tried to earn his praise was by developing the C-Virus. As it would turn out, she was nothing more than a tool more to him to accomplish his goals. The person Carla Radames ceased to exist when she was used against her will in Derek's experiment to recreate Ada Wong.
Carla appears in all four of the game's campaigns, though she is more prominently featured in Chris Redfield's and Ada Wong's. Chronologically, Carla first appears in Chris' campaign in Edonia and, under the guise of a hostage captured by the J'avo, leads Chris and his team into a trap injecting them all but Chris and Piers, who manage to escape, with a needle-grenade containing the C-Virus turning them into B.O.W.s.
Afterwards with the help of her creation Ustanak, she is able to capture Jake Muller and Sherry Birkin and relocate them to a Neo-Umbrella-run facility. During their 6 months imprisonment before they manage to escape, Carla used some of Jake's blood to create an enhanced strand of the C-Virus.
During those 6 months, Carla, then under the guise as Derek C. Simmons using a voice modulator, lures Ada into finding out about Carla's own existence and attempts to manipulate her into killing Derek. However, Ada soon uncovers Carla's plan, including Carla's desire to destroy the world - the world Derek and The Family have created. Starting with China, she unleashes the J'avo into Lanshiang. Using her enhanced C-Virus, she then sends one of her J'avo to infect Simmons.
Meanwhile Chris Redfield, after recovering from his Post-Traumatic Amnesia, goes on the hunt for Carla believing her to be Ada Wong. After finally managing to catch up, she ambushes what remains of Chris' team in Lanshiang and injects one of Chris' men, Marco, with the C-Virus. Once again Chris and Piers then manage to catch up to her once more, only for Leon and Helena to arrive also and protect her, believing that she is Ada Wong. Amidst the confusion as Leon and Chris are starring each other down, Carla throws a flashbang and escapes. Leon reluctantly agreeing that 'Ada' is a danger to society allows Chris to then carry on his chase and go after Carla.
After another cat and mouse chase, Chris and Piers finally catch up to Carla once more on a large ship. Only before Chris can arrest her, she is assassinated by an operative of The Family. Her body falls to the deck below and Chris assumes that she has died. The real Ada Wong, who was also chasing after Carla to stop her, then stumbles upon her corpse. Initially believed to be dead, Carla quickly begins to mutate after injecting herself with her last vial of C-Virus. Driven insane and believing that she is in fact the real Ada Wong, Carla rapidly turns into a shapeless white goo and envelopes the majority of the ship. Managing to fight her way to Carla's 'face', Ada then shoots through it eventually hitting tanks of liquid nitrogen which kills the now monstrous Carla completely.