Carys Fletcher was a Cardiff girl in her late teens or early twenties living at home with her father, until the sex Gas possessed her.
Biography
One evening during a club, after splitting up with her boyfriend, Eddie Gwynne, 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 3 captured her with a portable prison cell and took her back to the Hub for questioning until she persuaded Owen Harper 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, she returned to her previous life. (TW: Day One)