F.O.W.L. (short for Fiendish Organization for World Larceny) is a terrorist organization (similar to S.P.E.C.T.R.E. from the James Bond novels by Ian Fleming) originally introduced in the DuckTales episode "Double-O-Duck" as the "Foreign Organization for World Larceny".
The High Command - They are the head agents of F.O.W.L. It seems that throughout the entire series they never left their certain area (the area they were was possibly F.O.W.L. HQ). Also, they were aways hidden in the shadows, and their true names were never mentioned during the Darkwing Duck series run at all. The ones that usually talked were the one with a big head and fangs and the one with a hunched back, though not always with the same voices (in "Heavy Mental", the large-headed one is a woman). The one wearing the oversized hat never talked. It also appears that they were anthropomorphic birds. All members of F.O.W.L. are shown to fear them, as they appear to be prone to violently dispose of those who upset them, including Steelbeak's predecessor, who was put through six trash compactors.
Steelbeak - Acting like a 1920's gangster, Steelbeak is a rooster who is the top agent of F.O.W.L. He gets his name from the metal beak he wears in place of his original one which was apparently lost sometime before the events of the show, though it is not revealed how. This trademark beak also serves as a weapon because it can bite through almost anything.
Ammonia Pine - A former cleaning lady who worked at a research lab, Ammonia accidentally inhaled the fumes of an experimental bathroom disinfectant, which had a strange effect on her and turned her into a cleaning-obsessed villain. She was later recruited by F.O.W.L. for their dastardly plans. She hates dirt and is even afraid of it. She has a sister named Ample Grime, also a F.O.W.L. agent.
Ample Grime - Ample is Ammonia's messy, dirt-loving sister. No origin story is given for why she loves messes so much. She hates and fears cleanliness as much as Ammonia hates dirt, which proves to be a problem when the two are required to team up.
Major Synapse - The typical R. Lee Ermey-type military man. He once sent an Eggmen military unit into an active volcano so it could be "captured." He steals S.H.U.S.H.'s Norma Ray, using it to turn his useless hippie underlings into superpowered malcontents. He later used it on himself, turning into a giant head with a giant brain that had major telepathic abilities until his own stupidity caused his underlings Hotshot and Flygirl to be flattened by a cartoon anvil which they are last seen crawling away under. His own superbrain explodes when Darkwing Duck, Launchpad, and Gosalyn overloaded him with questions.
Hotshot - One of Major Synapse's hippie underlings changed by the Norma Ray. He controls both fire and ice.
Flygirl - One of Major Synapse's hippie underlings changed by the Norma Ray. She can move things with her mind and can also fly.
Eggmen - F.O.W.L. foot-soldiers, instantly recognizable by their white, egg-shaped helmets.
Quackerjack - A toy maker gone insane and wearing a harlequin-style jester costume, Quackerjack uses his arsenal of dangerous playthings as weaponry while he robs banks or just causes chaos for the fun of it. He is almost never without his trademark doll, Mr. Banana Brain, which can be used as an explosive. His most common toys are mechanical toy chattering teeth of various sizes. He has been known to team up with Megavolt. In the comics by BOOM! Studios, Quackerjack has become a more dangerous villain after Negaduck destroyed Banana Brain and told him he wasn't "mean enough" to be a serious villain: he's determined to show "the world he is mean enough".
Mr. Banana Brain - A banana-headed doll that Quackerjack seems to always have with him. Not really a character, but the closest thing he has to a recurring sidekick and frequently makes "talk" via squeaky-voiced ventriloquism. He doesn't realize Banana Brain is a toy and that he's doing its voice. The doll was possessed by the demon Paddywack in "The Haunting of Mr. Banana Brain". Negaduck deliberately destroyed him in the comic "The Duck Knight Returns", and Quackerjack has tried to create replacements.
Wolfduck - Wolfduck was created solely for the Capcom video game. He does not appear in the TV series or the comic book. Because of this, not much is known about him or his back story. Presumably, he was bitten by another wolfduck at some point in time and goes berserk when transformed.
Liquidator - Bud Flud, a slimy dog salesman who was in the bottled water business, was mutated into his present state, a water based monstrosity, after falling into a vat of a competitor's water which was revealed to be laced with a corrosive chemical during a battle with Darkwing and Launchpad. He can control all water. He constantly talks in advertising-style lingo. Since he is made of water, he and Megavolt do not interact well.
Professor Moliarty - A mush-mouthed mole who schemes to put the surface world into an eternal darkness. His name is a reference to Sherlock Holmes nemesis James Moriarty and his persona may be inspired by Marvel Comics' Mole Man.
Dr. Reginald Bushroot - A scientist who tried to fuse animal DNA with that of plants in an attempt to give people the ability to feed themselves through photosynthesis. The experiment was a success but with a price. Bushroot was turned into a half-duck/half-plant creature who now desperately seeks a friend or girlfriend. Bushroot uses his newfound control over plants as his main weapon and views them as people. He was the show's earliest recurring supervillain.
Megavolt - Elmo Sputterspark is a rat and was a high school nerd who gained the power to control electricity when one of his experiments was sabotaged. He's former high school classmate of Drake Mallard/Darkwing Duck, and his oldest villain: the two first battled at St Canard High's senior prom. Megavolt is quite psychotic and uses his control over electricity as his main weapon. His costume consists of his original jumpsuit, gloves, boots, and safety goggles from the power company as well as a giant battery worn on his back to keep his power flowing on the go and a headpiece that resembles a big plug. Megavolt was the show's creators' favorite villain and therefore the most recurring. He seems to regard light-bulbs and other electric apparatus as his friends and many of his crimes involve "liberating" them. Megavolt often referred to Darkwing as either "Darkwing Dork" or "Dorkwing". He shorts out when hit with water, a weakness that Darkwing often uses when defeating him. He was the first villain to learn Darkwing's secret identity but, being mad, forgot what it was by the end of the episode. In the comic story "The Duck Knight Returns", he was forced to briefly retire from supervillainy and get an office job (where he worked next to Drake); he found he preferred prison to working at Quackworks.