Ng

"Ng" was the name used by a being also known as the Herald. She prepared planets for the arrival of God.

Time as the Herald
Ng acted as the Herald for God and would travel from world to world spreading the word of God and preparing the inhabitants for the arrival of God. (AUDIO: Future Pain)

Arriving in Cardiff
When the Sorvix fled Sorva through the Rift, Ng followed them and arrived in Cardiff during the time 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)

Ng arranged to meet Tyler Steele, hoping to recruit him into Torchwood, however he turned the offer down after seeing how disorganised it was. She then asked Mr Colchester to visit Tyler, in an attempt to change his mind. Ng met with Tyler at Cardiff Bay following his escape from 3Sol with Jack. (AUDIO: Changes Everything)

She killed Gwen's mother Mary Cooper (AUDIO: Aliens & Sex & Chips & Gravy) and other individuals who suspected that she was not 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, 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)

Ng was imprisoned in the Hub (AUDIO: Future Pain) before being released by Yvonne Hartman to work for Torchwood. Jack refused to associate with her because of Gwen's departure being her fault. (AUDIO: See No Evil, Night Watch) She was almost attacked by the predator but God sent it away. (AUDIO: See No Evil)

Behind the scenes

 * The character mostly pretends to be Gwen Cooper and, hence, is never named in the Aliens Among Us audios. "Ng" (pronounced "eng") is the way she was presented in the cast lists. In interviews, it was mostly treated as an abbreviation of "not Gwen" and at least once as "new Gwen". The name (with this pronunciation) is used in-universe starting from God Among Us 1.
 * 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 Davies 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)