You will first come across him in Avestreu (during the quest The Diadem And The Lover), where he enables you to use his social and thieving talents, even if you don't have any yourself. As part of the story he leaves the party again shortly before entering Ferdok (so you should take away all important or valuable items that he is carrying, before leaving Avestrue), but you can get him back by means of the quest Salina's Request after solving the Murders in Ferdok. (Note: In my game I had to go to the Ardo's residence first after completing the Murders in Ferdok quest. After talk with Caretaker_Muddlemosh Salina appeared on the square.)