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An alien fled from the Sorvix's God to Cardiff at the time when Torchwood Three was rebuilt by Gwen Cooper. She took over her body and posed as Gwen for a couple of months, raising only slight suspicion that "something was off" from Jack Harkness and Mr Colchester. (AUDIO: Herald of the Dawn)

She killed Gwen's mother Mary Cooper (AUDIO: Aliens & Sex & Chips & Gravy) and other individuals who suspected that she wasn't who she seemed to be. (AUDIO: Tagged) While possessed, the real Gwen's consciousness unsuccessfully tried to communicate with the others. Only the alien could hear her. (AUDIO: Aliens & Sex & Chips & Gravy)

While investigating an escape room with Rhys, Mr Colchester and his husband Colin Colchester-Price, she raised more suspicions from Colchester for her rough decisions. (AUDIO: Escape Room, Herald of the Dawn) Eventually Andy Davidson contacted Jack when "Gwen" started to behave strangely upon hearing the proclamations "God is returning" from random people. Jack faced "Gwen" at the Hub, where Mr Colchester tried to contain the unstable Rift. Jack asked him to leave it as it is, and a burst of energy from the Rift separated the real Gwen from the alien. (AUDIO: Herald of the Dawn)

Behind the scenes

 * James Goss and Scott Handcock said that the character was pitched by Russell T Davies, who helped to develop Aliens Among Us for Big Finish Productions. It was the only way to include Gwen Cooper in Aliens Among Us due to the unavailability of Eve Myles for most of the recording sessions for the ongoing series. However, Eve recorded a few lines in a single day, which were used throughout the series, and the final monologue (written by Russell himself) for the final episode. While unavailable for the ongoing series, Eve returned to play Gwen for many standalone stories for Big Finish. (BFX: Aliens Among Us 3)
 * According to Goss and Handcock, Alexandria Riley's performance as Gwen "fooled quite a few people", who, upon listening to the first episode of the series, thought that it was Eve Miles playing Gwen. (BFX: Aliens Among Us 3)
 * Russell T Davies suggested that "Not Gwen" should kill Andy Davidson, but Goss and Handcock felt it would be too much, as the character is very much loved by the whole team and the fans, and replaced Andy with Gwen's mother. (BFX: Aliens Among Us 3)