Joker

Joker is a villain that appears in Ben 10: Forces United and Ben 10: New Age and is a common foe of Batman

History
In this series he was only referred to as simply "Joker" and rarely as "The Joker" - he had wild green hair, red eyes, dark red lips, unusual yellow teeth, and a milky blue tongue. He was born by falling into a vat of chemicals, which bleached his skin, dyed his hair and drove him to insanity. In his first few appearances, Joker's costume consisted of a purple-sleeved straitjacket, blue pants, finger-less gloves, and bare feet. Later in the series, Joker wore a purple jacket, orange shirt, and dark purple leggings( with finger-less gloves of the same material), but still had wild hair and wore no shoes, except one episode (which had mechanical stilts concealed in them) in which ironically he reverted to his earlier costume. Joker also moved and fought with a monkey-like style, by using his feet as dexterously as his hands, and often hung from the walls and ceilings.

While it could be argued that this version of the Clown Prince of Crime more resembled the colorful, comic relief character featured in the comics of the 1950s and '60s, he was still a highly dangerous, intelligent, and cruel character - while most of his crimes were silly and prank-like in nature, they could still be legitimately destructive. Joker treated each of his crimes like an elaborate joke designed to make his victims literally die laughing. Due to his unpredictable nature, bizarre appearance and unnerving instinct for the absurd, Joker earned a reputation as one of the most dangerous criminals in Gotham.

The Joker in that series had often switched from a prankster-like clown to a much darker murdering psychopath (as in later episodes). Within The Batman, the Joker's body count could be arguably ranged from 19-30 victims on screen.

Joker believed himself to be inextricably linked to the Batman, and considered him to be his favorite sparring partner and opponent. Joker said that their relationship was "like comedy and tragedy - two sides, same coin". Joker's weapons included his signature Joker Venom (called "Joker Gas" in the series), razor sharp playing cards, grenades with Joker smiles on them, and a bomb shaped like a Jack-in-the-Box. One of his most famous crimes was creating the supervillain, Clayface. Joker captured and tortured Detective Ethan Bennett, and, in order to drive him mad, he exposed Bennet to "Joker putty" a deadly chemical that he created which mutated Bennet and transformed him into the shapeshifting monster, Clayface. As Clayface, Bennet attempted to get revenge on Joker many times. It was possible that this incarnation of the Joker had an origin similar to the Killing Joke, as he mentioned that he had always wanted to make people laugh. In a flashback to his origins, Joker was shown wearing red clothes after he emerged from the chemicals which suggested at the very least a Red Hood origin. It was also hinted that Joker would one day cripple Batgirl like in "the Killing Joke" as Barbra Gordon was shown to be confined to a wheelchair in the future. He also maintained a rivalry with the Penguin for the title of Gotham's greatest criminal. In "The end of Batman" was the last episode when Joker and Harley Quinn appeared and they both escaped.

Ben 10: Forces United
Joker first appears in Don't Laugh where he escapes prison along with many other villains and faces Ben and Batman multiple times until he is ultimately defeated

He then appears in The Fearsome Five having escaped the police and joins up with Penguin, Mr. Freeze, Bane, and Killer Croc to kidnapp Max Tennyson and lure Ben out so that they can crush the new hero but are all defeated and sent back to jail.