King Julien is a ring-tailed lemur who has light gray and white fur, big yellow eyes, tiny whiskers, a black nose, thin black eyebrows, fox-like ears, a long black-and-white striped raccoon-like tail and wears a crown.
Despite his desire for everyone to listen and do what he wants, King Julien hates and ignores Mort, who treats him like a god. Julien hates having anyone touch his feet (such as passing a royal decree that anyone, who touches his feet would be banished from his kingdom). He loves to dance and likes to play the music loud. He also likes to annoy Skipper and the other animals. He's mostly selfish, hyper, romantic, lazy, and will try to his ideas instead of others. He also likes to dance and have tons of fun with the lemurs and the other creatures of the jungle.
Julien ruled over a great colony of lemurs in Madagascar with charisma and certainly little ability to lead. It is a goof for King Julien to be a king since male ringtail lemurs are not leaders (although in the movie, Maurice said that he was "self-proclaimed" lord of the lemurs). But then, Julien is not really a very good king. King Julien's main skill lies in delegating; telling other people what they should do.
Only Mort and Maurice truly believe and respect King Julian as their king; everyone else just humors him. Mort responds to everything, that King Julien tells him to do (as "I like...", followed by whatever King Julien said).
Julien had parties for his people on Madagascar with much frequency, the likes of which were interrupted constantly by the fossa.
The day that Alex, Marty, Gloria, and Melman came to Madagascar, King Julien and his subjects were enjoying one of their parties when it was invaded by the fossas. When the fossas were scared off by Alex, Julien and his people were hiding in the trees and saw this scene, thinking that Alex and his friends were giants from some faraway land, with Julien coming forward and proudly presenting himself as the King of the lemurs.
When asked where the people were, King Julien pointed to a skeleton of a man caught in a parachute hanging from a tree. Later that night, Julien had a meeting with the lemurs and the other creatures of the jungle in the plane. He insisted that if they could make friends with Alex and his friends, the fossas would be scared away for good. King Julien's second-in-command Maurice was skeptical about this plan, but Julien insisted that his ideas were the best since he's the King.
When Alex and his friends were ready to go back to New York, King Julien gifted them his crown; but he already made himself a bigger, better crown.
In Madagascar: Escape 2 Africa, King Julien, Maurice, and Mort accompany Alex, Marty, Gloria, and Melman to New York on the plane, built by the Penguins. He's in the 1st class, ordering constant refreshments, and amusing himself with plane crash videos. As the plane crashes down to Africa after running out of fuel and momentum, Julien takes utter pleasure in the feeling of weightlessness, caused by the high speed descent. By being in 1st class, King Julien escapes the crash with a parachute.
Upon arriving in Africa, King Julien at first believes that they're back in New York, and he makes himself a new crown. Later, when the reserve watering hole dries up, Julien suggests that they make a sacrifice to "his good friends, the Water Gods" at the volcano to replenish the water. At first, Julien suggests Melman as the sacrifice (who believed that he was gonna die soon anyway). But after Melman realized that he was gonna live, he gave up on being a sacrifice, leaving King Julien disappointed at his plan's failure. But Mort arrives soon after, followed by a persistent shark that had been trying to eat him. The shark then falls into the volcano, and as King Julien comes out of the volcano, he sees the water coming back to the reserve (because of a dam, constructed by Nana being destroyed by Alex and Zuba), and he's convinced of his plan's success in the end.
In the episode, "Crown Fools", King Julien panics when he loses his crown and obsessively longs for it, until it is revealed that he had a spare crown all along, prompting Marlene (who was trying to make Julien new crowns all day) to chase King Julien with a crowbar.
When the sewer rats went to the zoo and tricked the Penguins out of their home, King Julien decided to play for the Penguins in a game of Hockey to try and win their home back. They were losing, until one of the rats touched Julien's feet, after that he lost it and beat all of them on his own.
In the episode, "Haunted Habitat," King Julien mentioned that he had a dream that he was "the last mammal on Earth." Mort was just happy to be in the dream (despite that he was "roadkill" in it).
In "Misfortune Cookie," Rico gets a fortune cookie, that reads, "You will soon meet a fowl end." The Penguins deny that fortune cookies have any power (despite getting 3 accurate fortunes), and dismiss it as superstition. King Julien exclaims that "these stitions are super," and attempts to get the Penguins to believe in the supernatural. King Julien actively plots to make a "fowl end" fortune come true -- by his own doing.
A recurring gag in The Penguins of Madagascar series is that King Julien's behavior and believing in Supernatural Beings, a belief, that annoys the other animals.
It is also shown that King Julien gains a super-lemur level of strength if he gets mad. Visible when in "Miracle On Ice," King Julien spanked and humiliated the Rat King, after one of his henchmen touched Julien's feet; in "Happy King Julien Day," Maurice claims that King Julien violently used a cane to spank everyone, who forgot his self-proclaimed holiday.
In "Dr. Blowhole's Revenge," it's revealed that King Julien's a double agent for the Penguins & Blowhole.
In Madagascar 3, King Julien, Alex, Marty, Melman, Gloria, Mort and Maurice are willing to go back to the Zoo, but the penguins left Africa without them on a super-plane with the chimps. They decide to go after them, to Monte Carlo. There, Chantal Dubois from Animal control is introduced. She chases the animals on a motorbike and shoots King Julien with a sleep dart. The animals get away on the super-plane but crashes in a train site. They run away with ta circus which is heading for rome. There, King Julien meets his true love, Sonya the bear. When the circus show is about to start, Stefano the Sea-Lion can't find Sonya and Julien. It turns out that Sonya and Julien are on a romantic date in Vatican City. Sonyas act is tricycle riding. King Julien and Sonya share the act later on in the movie.
In the special, King Julien finds a potion and thinks he makes anyone be irresistible, King Julien cheats animals with a trick in which Maurice was dressed as a woman and was in love with Mort with potion. Later, the potion wears off and he asks for penguins redo it after Julien cheats Alex creating a lotion that humiliates Alex, in the end, he created another potion that just by eliminating his fur.