Mundungus Fletcher is a wizard and one of the original members of the Order of the Phoenix, a group dedicated to fighting Lord Voldemort and his Death Eaters. He is somewhat of a con artist, if not an outright criminal. He was not well-liked by the other members of the Order and struck many as seeming untrustworthy. However, he was fiercely loyal to Dumbledore after Dumbledore helped him out of a "tight spot." His links to the wizarding underworld are extremely useful in gaining information for the Order.
Squat, with bandy legs and long, straggly ginger hair, Fletcher often sported an unshaven face and bloodshot, baggy eyes. He had a habit of smoking a smelly pipe and carried a strong bodily odor of tobacco and stale alcohol. He was once even mistaken for a pile of rags.
His loyalty to the Order did not prevent him from pursuing his criminal ways during the period before Voldemort's return, however. In 1992 he tried to place a hex on Arthur Weasley during a law enforcement raid, and in 1994, following the Death Eater attack on the Quidditch World Cup, he put in a compensation claim for a twelve-bedroomed tent with an built-in Jacuzzi (in actual fact he was sleeping under a cloak propped up on sticks).
With the return of Lord Voldemort in 1995, Dumbledore called upon Sirius Black to alert the 'old crowd', which included Mundungus Fletcher. However, Fletcher found it impossible to place the demands of the Order completely ahead of his own interests.
His main tasks for the Order during the school year of 1995-96 were those which involved keeping a close eye on Harry Potter. The consequences of failing in this mission were made known at the beginning of that year, when Fletcher abandoned his post to smuggle stolen cauldrons. In his absence, Harry and his cousin Dudley Dursley were attacked in Little Whinging by Dementors under the command of Dolores Umbridge. He later redeemed himself by keeping a close watch on Harry, during his times in Hogsmeade Village, and reporting back to the Order. Specifically, he disguised himself as a witch under a black balaclava in the Hog's Head Inn.
These good intent behind this gesture was disregarded by Harry when, following the death of Sirius Black in 1996, Harry caught Fletcher red-handed in Hogsmeade, selling items which had been stolen from Sirius' house. These items and the entire residence at Number 12, Grimmauld Place were left to Harry following Sirius' death. Harry confronted him using physical force before Fletcher Disapparated. His exact whereabouts continue to be a mystery.
One of the items Fletcher stole was in fact the locket that R.A.B. stole from Voldemort. By the time he was found however, the locket had already made its way into the hands of Dolores Umbridge.