Carys Fletcher was born on 13 November 1987. She took up swimming at least when she was six. Her mother died in a car crash in 1997. In 2006, most of the emails she sent were about Orlando Bloom and Heath Ledger. At around this time, she worked as a temporary secretary at Conway Clinic.
In 2007, Carys had split up with her boyfriend, Eddie Gwynne, over the phone. At a nightclub, the sex gas entered into Carys's body and used her as its vessel. Taken over by it and now able to generate powerful pheromones, she seduced and killed a young man at the club named Matt. The gas fed by stimulating its victims to the point of orgasm, at which stage their bodies exploded into dust.
Torchwood Three captured her with a portable prison cell and took her back to the Hub for questioning. Gwen Cooper attempted to talk to her, but was seduced by the pheromones and made out with Carys until the sex gas realised that it needed men. Later, she persuaded Owen Harper (again by seducing him) to release her. She eventually killed her ex-boyfriend, as well as several clients at the sperm bank where she worked, before Torchwood caught up with her again. After the team managed to separate the gas from Carys, they took her home to her father. She was then able to return to her previous life, seemingly with no or hazy memories of the events that the possession had caused. (TV: Day One)