"Professor" Cranberry is the Assassin hired by Cosmo Lavish, who cleans up the loose ends that could lead to Cosmo's involvement in his plot to replace Vetinari; rings made of Stygium, stolen shoes and swordsticks.
Despite the honorific, Cranberry is not a Professor of the Assassins' Guild, though he had been a scholarship boy there in his youth. His title derives from the fact that, when not out assassinating people he spends his time reading. This hobby adds to the general revulsion toward his character; Cranberry is in no way an illiterate thug but a well-read, highly intelligent man who takes his job seriously and efficiently - to the delight of Cosmo and the horror of Heretofore, Cosmo Lavish's servant. Heretofore has a few deaths on his hands, because his schemes to con Cosmo out of money end with Cranberry being sent in to inhume whomsoever helped Heretofore out. He meets his end at the hands of Mavolio Bent, who killed him in a most unusual way. For details, see Making Money.
Cranberry is patterned after the kind of stereotypical "cleaner" figure found in many gangster movies, TV shows and novels of that genre. In the French film Nikita by Jean-Luc Besson, there is a character called Victor le Nettoyeur (Victor the Cleaner), played by Jean Reno who is sent in by the French Government to erase any mistakes they may have made, killing anyone who may have seen anything untoward. Harvey Keitel reprises this role in the American remake and then goes on to play Winstone "the Wolf" Wolfe in Pulp Fiction who does a similar role cleaning up "problems".