The Gentleman (Escape to LA)

"The Gentleman" was the name taken by an assassin tasked with killing Jack Harkness.

Biography
The Gentleman followed Esther Drummond to Sarah Drummond's house. From there he managed to track her and Jack to Los Angeles where he identified Gwen Cooper as an associate of his target.

Knowing he would need his biometrics to follow Jack, the Gentleman attacked Nicolas Frumkin and took a hand and an eye from him as well as a recording of him saying his name. With this, he followed Jack into the PhiCorp building and captured both him and Gwen. He was fascinated by Jack being the last mortal man and was about to kill Gwen when Rex Matheson shot him multiple times, once in the neck, preventing him from speaking. (TW: Escape to LA)