Ok people, I have worked out who Sparrow is...
He's Gino. Think about it for a second, Gino would know where Bones and Shroud are MOST of the time, so he could always track them down, and he wears orange, like Sparrow's trousers, plus I think this is the thing that Volrath and ArtBane have been hiding...
Now you may be blurting out "HE CAN'T BE 'CAUSE HE WAS WITH STOIC AND THE BARON WHEN SHROUD WAS HORSE RACING SPARROW (-ish)" But he was really just duplicating himself. He did this by seeing another great mage duplicate himself, and so he can therefore carry out said magical abilities.
AND also, what "friends" of Gino's was he visiting in Boreal?
It's a strech I know, but I think that's who he is.