Muggles' Guide to Harry Potter/Characters/Willy Widdershins

Overview
Willy Widdershins is apparently a small-time hood and mischief-maker.

Order of the Phoenix
Mundungus Fletcher, at the first dinner at the Headquarters of the Order, is overheard regaling the twins and Ron with a story about a large quantity of toads he nicked off one "Will" and then sold back to him. Though "Will"'s last name is never mentioned, it is possible that this is Willy Widdershins.

Just before Harry's hearing at the Ministry, Arthur Weasley mentions that he is trying to track down a prankster who is charming public toilets in Bethnall Green to make them regurgitate.

When Harry and the Weasley family visit Arthur in St. Mungo's Hospital on the afternoon after the snake attack, Arthur has evidently found an article in the Daily Prophet saying that Willy Widdershins had been responsible; one of his curses had backfired and he had been found in the toilets covered head to foot in... It appears that somehow he got off being charged; Arthur surmises that gold changed hands. Arthur comments that he's since been caught selling biting doorknobs to Muggles, and that two of them had lost fingers and were in St. Mungo's having bones regrown, and seems rather hopeful that he'll actually see them.

At the meeting in Professor Dumbledore's office after Marietta Edgecombe has told Dolores Umbridge about Dumbledore's Army, Umbridge reports to those present that her informant, Willy Widdershins, was at the bar in the Hog's Head when the first meeting of what was then called the Defence Association was held. Although he was heavily bandaged, his hearing was still fine, and he immediately reported what he had overheard to Umbridge. Professor McGonagall, on hearing this, says that now she understands how he got off so lightly for all the regurgitating toilets.

Analysis
"Widdershins" is an old English word meaning "backwards". This may be deliberate on the part of the author, insofar as suggesting the character's morals are backward, or it may be coincidental, as "Willy Widdershins" is just a playful alliterative name.