The Gentleman (Escape to LA)

"The Gentleman" was the name taken by an assassin tasked with bringing Jack Harkness to the people responsible for the miracle, presumably the three families.

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 threatening to shoot Gwen when Rex Matheson shot him multiple times, once in the neck, preventing him from speaking. Unfortunately, this occurred just as he had been about to reveal the names of the mysterious individuals responsible for the Miracle. (TW: Escape to LA)

Being a category one deceased, he was presumably sent to the modules and incinerated. (TW: The Gathering (TV story))