James Mayer

James Mayer was an alien from another dimension who had integrated himself into the Torchwood team.

Biography
To further understand the flotsam and jetsam that fell through their version of the Rift, the Border Princes fully integrated themselves into their host dimension, to the point where they fully believed they belonged there. Fail-safes were in place to remind the Border Princes of their true nature in case of damage or other failure. However, this failed with James. His bodyguard subsequently needed to take steps to rectify the situation, which unfortunately meant the ‘death’ of James’ earthly body.

The 'camouflage effect', which made the team think they knew James, only had a 100 mile radius, which was discovered by Gwen during a trip from Cardiff to Manchester.

Ultimately the team had to come to terms with the fact that a valued member of the team, as well as a best friend to each, did not actually exist. This especially affected Gwen, who had been conducting an affair with James and was about to leave Rhys. (TWN: Border Princes)

Trivia

 * This wasn't the only time Torchwood 3 has been infiltrated in this way, as a being called Adam Smith later tried something similar before being discovered. (TW: Adam)