I could give a whole kerfuffle of text on his backstory and what he does etc. etc, but everybody at this point knows who he is.
Jaqen is a faceless assassin. A sect of people who worship death, and provide it in return for the thing you value most. (you must value you the persons death more than anything else in other words)
His description is identical to that of the Alchemist (Hooked nose, Black curly hair, scar on right cheek)
He uses a citadel apprentice to steal a lead key which supposedly opens every door in the citadel.
His name; the Alchemist is a pun (The lead key he trades for a gold coin - lead into gold, Alchemys ultimate pursuit)
I dont care to speculate what he is after, but the unusual thing that jumps out at me, is that Jaqen breaks faceless assassin rules. He is only allowed to kill those who do not tell him their name, Jaqen actually asks Pates name, then kills him afterward (presumably poison)
Pate is perfectly fine and talking to Samwell Tarly at a later date in the story when Sam goes to the Citadel, but Sam has a strange dislike to him.
Why would Jaqen break the rules of the Faceless contracts? And they do not accept Contracts which do not result in the death of someone, It sounds like he is rogue, or serving an ulterior motive.
Sidenote: Id like to see Arya and Jaqen meet again, both rogue, or one trying to kill the other.