A unique Skin Thief
Pimply was an awkward orcish busboy in the Big House. By some unknown criteria, the Skin Thieves decided he would be the perfect cover, killed him, and replaced him with one of their own. This may have been the end of Pimply, but some change came over the Skin Thief that replaced him. The exact nature of the change is not know, but this Skin Thief can now appear as Pimply without having to wear his skin. Even more fascinating, it seems to have picked up a great deal of the personality and mannerisms of the original Pimply, to the point where it self-identifies as him now.
Despite (or perhaps because of) being willing to help his former targets (Harrik, Ssama, and the heroes) against his brethren, Harrik has been keeping him imprisoned in the Big House. This is is probably smart, given that he has on several occasions talked about a compulsion from that urged the Skin Thieves to evolve and carry out their sinister missions.
In fact, most of the information you have about Skin Thieves motivations come from interactions with Pimply.