The Fiendish Five was a group of criminals responsible for the murder of Sly Cooper's father and the theft of the Thievius Raccoonus. Ten years after the incident, Sly embarked on a mission to defeat the Fiendish Five, avenge his father, and reclaim the book.
The Fiendish Five was formed by Clockwerk for the sole purpose of wiping out the Cooper Clan, whom he had hated for thousands of years. When Sly Cooper was eight years old, the Fiendish Five attacked the Cooper home, killing Conner Cooper and stealing the Thievius Raccoonus. They split the book into five pieces and left Sly orphaned. Clockwerk, although aware that Conner had a son, left Sly alive because he believed that he would not be a threat without the book.
After the attack on Sly's home, the Fiendish Five went their separate ways, each one with their own schemes in mind. Raleigh began the legend of the "Welsh Triangle" by creating a storm machine to sink ships around the Isle of Wrath. Muggshot took over Mesa City, slowly driving out the locals and turning the city into his own personal gambling empire. Mz. Ruby withdrew from civilization entirely, preparing an army of voodoo creatures to unleash terror on the world. The Panda King returned to China to take out his aggression on the countryside that spurned him with an avalanche-based extortion scheme. Clockwerk headed to Russia to begin his own plans.
Ten years later, Sly Cooper, now partners with Bentley and Murray, began tracking down the members of the Fiendish Five. In the process, he retrieved the stolen pages of the Thievius Raccoonus. After the rest of his cohorts had been defeated, Clockwerk himself was confronted by Sly. He made several failed attempts to kill the Cooper, but in the end, was defeated as well. With Clockwerk dead and the rest of the members incarcerated, the Fiendish Five ceased to exist.
- Clockwerk (founder/leader; deceased)
- Muggshot (enforcer; incarcerated)
- Mz. Ruby (chief mystic; incarcerated)
- Panda King (demolitions expert; now member of the Cooper Gang)
- Raleigh (chief machinist; incarcerated)
Each member of the Fiendish Five employed a number of guards to protect and patrol around their various facilities:
- Sir Raleigh
- Mz. Ruby
- Water rat
- Giant Mosquito
- Blue Widow
- Swamp monster
- Ghost rat
- Demonic Turtle
- Bomb Toting Rooster
- The Beast
- Panda King
- While Clockwerk, Panda King, and Muggshot all reappeared in the two sequels at some point (Clockwerk was also in Sly 2: Band of Thieves, and Panda King and Muggshot were in Sly 3: Honor Among Thieves), Sir Raleigh and Mz. Ruby have yet to reappear. Although this could easily be explained by the fact that they simply never escaped jail.
- The Fiendish Five may be a play on to the Fearsome Five, a DC Comics villain group and/or the villain group of the same name from the Darkwing Duck television series.
- Portraits of the Panda King, Muggshot, Mz. Ruby and Sir Raleigh all appeared in the Cairo Museum in Sly 2.
- In the Sly Cooper and the Thievius Raccoonus credits, Mz. Ruby was the only member of the Fiendish Five to be credited in the "Voice Actors" section.
- Unlike the Klaww Gang, the Fiendish Five had no unified master plan. Each was only out for their own gain.
- Each member, except Clockwerk, comes on the loudspeaker twice in their respective episodes to speak to their guards. Clockwerk does not do this probably since he does not have regular guards like the other members do.
- The Fiendish Five is the only villainous organization that does not appear to have a personal insignia.