Torchwood actor John Barrowman has joined Arrow. Barrowman will board the network's adaptation of the DC Comics series as a “well-dressed man” who is “as mysterious as he is wealthy … he is an acquaintance of the Queen family and a prominent figure in Starling City.”
Barrow is no stranger to the genre, having auditioned for Superman in Superman Returns and voicing a desire to portray Captain America at SDCC '09. Barrowman has a quasi-cult following from the BBC sci-fi drama, Torchwood.
If I had to make an educated guess, I'd say he's portraying a DC hero but producers are playing their cards close to the chest.