The villain in “The Adventure of the Six Napoleons” is Beppo.
Beppo is an Italian criminal who hides the Black Pearl of the Borgias inside one of six plaster busts of Napoleon. He originally steals the jewel and conceals it in a freshly made bust at the factory where he works. After serving a prison sentence for stabbing a man in a street fight, he tries to recover the pearl by tracking down every bust from the same mould.
His search explains the apparently senseless crimes that attract Inspector Lestrade’s attention. Busts are stolen from shops and homes, then smashed so Beppo can inspect the fragments. The case becomes more serious when Pietro Venucci, associated with the Mafia and connected to the pearl, is found murdered.
Holmes eventually identifies Beppo from a photograph and information gathered at the bust factory. He arranges surveillance around the final bust, allowing Lestrade and him to catch Beppo in the act. Beppo refuses to explain himself after arrest, leaving Holmes to reconstruct the entire plot by breaking the last bust and discovering the hidden pearl.
Charles Augustus Milverton, Professor Moriarty, and Colonel Sebastian Moran are important Holmes villains, but none is the culprit in this story.