The hereditary porter at the Assassins' Guild. His father was porter there, as was his grandfather. Stippler has probably seen it all by now, and if he hasn't, his father probably did. The few bits that neither of them have seen, we can only guess at.
Stippler is suspected of being an occasional source of information for the Ankh-Morpork City Watch, but one thing he will not divulge to anyone is which Assassin is currently out of the premises on an active contract: he knows, alright, as he not only guards the door, he does the internal post and puts the contracts into the named pigeonholes for collection. But some information may not be divulged: he prides himself on this. He will, however, scrupulously stay on the right side of Samuel Vimes - who is, after all, the owner of the building and like any landlord has an interest in knowing who's in his house - by adding the name that goes with the physical description of an assassin.