Answer:
Chris Keel, London. American. CI5-Agent (before: Navy SEALS, United States of America ) 
 

Evidence:
Declaration of Chris Keel himself. Also Keel and Curtis go actually once to Chris flat, which is overlooking a graveyard. See: "Back to Business"/"Der Bombenleger" 27.03.99 

Further Evidence: (especially refering to his past) 
"As an aside, I noticed he didn't have his SEALS insignia on his uniform during the wedding scene, but did in the shot in the credits, so I'm surmising it was his wife's death that drove him to join the SEALS and go on all those dangerous missions that won him the medals and citations - but I guess we'll never know." see: "Back to Business"/"Der Bombenleger" 27.03.99, eye-witness Jill Woolfenden, protocolled 8. Februar 2000 

Comment:
Strange, this Chris Keel. What do we see? We see the nice, young American. Friendly, easy. Then here is his flat - first shock: The Graveyard behind the window. And actually, it is a very big window, in his living room - Living with the dead?
Then, there are the photographs - standing here and also in the bedroom: Chris in uniform, with a pretty blonde close to him. Sam is surprised: Such a pretty sister? No. She was his wife, Chris answers friendly. Was? But the easyness vanishes completely, when we witness, what the american boy is seeing in his dreams: An innocent scene starting with the happy, just married couple, turning suddenly into something terrible: Shots, coming from nowhere, destroying forever the happy moment... The ultimative nightmare, it seems, is the other, very much hidden side of the sunnyboy. So the question arises for me in another, darker form again - not "What do we see" but: What are we allowed to see...
rg 12. Februar 2000