Capt. Jack Harkness (John Barrowman)
Jack is actually a Time Agent - part of a kind of space CIA - and he's trying to find two years of his memory that have disappeared. He's a rogue Time Agent and he knows he's done something in his past and he's not sure what it is or whether it is good or bad because his memory has been erased. He's also an intergalactic conman and he starts off by trying to con The Doctor and Rose. He tries to sell them something in order to get money because that's what he does. He has conned a lot of people in the past. His method is to sell people things that are not what he says they are - and then once he has got the money he runs.