King Julien XIII is a conceited, selfish, fun-loving ring-tailed lemur.
As a self-proclaimed lord of the lemurs, Julien ruled over a great colony of lemurs in Madagascar using mostly his charisma and delegating, i.e. telling other people what they should do since he certainly had very little ability to lead. Nevertheless, he shows more intelligence and calmness compared to the other lemurs (aside from Maurice), which is shown when the very word "Fossa" drives all of them, except himself and Maurice, into a state of panic.
He had parties for his people on Madagascar with much frequency, the likes of which were interrupted constantly by the fossa, which trespassed onto the lemur territory, devouring all that they could catch.
In Julien's early life as seen in All Hail King Julien, he was depicted as the nephew of Uncle King Julien XII, who banned all noisy things to keep the fossa from attacking. After Uncle King Julien XII abdicated to evade a prophecy from being eaten by the Fossa, Julien became the new King of the Lemurs and did things that were opposite his uncle's rules. He saves his people from the Fossa and got bitten on the butt by the Fossa and he survived. When he learns that his nephew is still alive and is still king, his devious uncle tries to get rid of him and reclaim the throne. When he returned, Uncle King Julien tricks Julien into going into the Fossa territory to find out what they're planning with Maurice joining him. In the end, Julien discovered his uncle's plot, glad that Clover chose him over his uncle, and gives his uncle a second chance, by putting him in the Fossa disguise, that is being cuddled by a Fossa girl, much to his uncle's dismay.
The day Alex and his friends came to Madagascar, King Julien and his subjects were enjoying one of their parties when it was invaded by the fossa. When the fossa were scared off by Alex, Julien and his people were hiding in the trees and saw this scene, thinking Alex and his friends were giants from some faraway land, with Julien coming forward and presenting himself as the king of the lemurs — once he had ascertained they were not dangerous by tempting them with Mort.
When asked where the people were, Julien pointed to a skeleton of a man caught in a parachute hanging from a tree. Later that night, Julien had a meeting of the lemurs and other creatures of the jungle in the plane. Julien insisted that if they could make friends with Alex and his friends, the foossa would be scared away for good. Maurice was skeptical about this plan, but Julien insisted that, being king, his ideas were the best. Julien's plan failed when Alex started to get out of control and try to kill the lemurs and his friends. Although, Alex then turned back to normal and the foossa were scared away with only the actions of the four main characters and the penguins, Julien took the credits for its success. Interestingly though, when it went wrong, he did not foist the idea back onto Maurice.
When Alex and his friends were ready to go back to New York, Julien gave them his crown; he had already made himself a bigger, better one using a leaf-tailed gecko named Stevie.
In Madagascar: Escape 2 Africa, Julien, along with Maurice and Mort, accompany Alex, Marty, Gloria and Melman to New York on the plane built by the penguins while leaving Stevie in charge until the day he returns. He is in the first 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 first class, he escapes the crash with a parachute, but loses his crown.
Upon arriving in Africa, Julien at first believes they are in New York, and makes himself a new crown. He manages to gain some power while in Africa, although very few characters actually pay attention to him, getting to ride a flock of flamingos with Maurice, then moving on to riding ostriches and finally an elephant. Later, when the reserve's watering hole dries up, Julien suggests to the animals that they make a sacrifice to "his good friends, the water gods" at the volcano to replenish the water. To convince the masses, King Julien enacts his casual conversation with the Water Gods. Later, Julien persuades Melman to volunteer for the sacrifice, who believed he was going to die soon anyway. But when Melman realized he was going to live, he gave up on being a sacrifice, leaving Julien disappointed at his plan's failure. Mort arrives soon after, followed by a persistent shark that had been attempting to eat him. This shark then falls into the volcano, and as Julien comes out of the volcano, he sees the water coming back to the reserve (due, in fact, to a dam constructed by Nana being destroyed by Alex and Zuba), and is convinced of his plan's success. From this King Julien concludes that the Water Gods like sea food more.
Julien appears in The Penguins of Madagascar regularly, often noisily and unintentionally annoying Skipper and pretty much everyone. He is the self-proclaimed "King of the Zoo" no doubt due to his former king status on Madagascar. Only Mort and Maurice truly believe and respect him as their king, the rest just show up to his "proclamation of royal decrees." Julien is proven to be a worthy ally in most episodes, since it is he who sometimes saves the day for the penguins (like in "Dr. Blowhole's Revenge"). 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 (the zoo). When the sewer rats went to the zoo and tricked the Penguins out of their home, 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 his feet, after that he lost it and beat all of them on his own.
In the episode "Crown Fools", 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 him new crowns all day, to chase him with a crowbar). Despite his desire for everyone to listen and do what he wants, he dislikes and ignores Mort, his most loyal follower, who treats him like a god. Mort responds to everything Julien tells him to do as "I like..." followed by whatever Julien said. In the episode "Haunted Habitat" he 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 "road kill" in it.
A recurring gag in The Penguins of Madagascar series is that Julien believes in supernatural beings, which sometimes annoys the other animals. In the episode 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 receiving three accurate fortunes, and dismiss it as superstition. Julien exclaims "these stitions are super," and tries to get the penguins to believe in the supernatural. King Julien actively plots to make a "foul end" fortune come true - by his own doing. It is also known that his believing in spirits can be used against him, seeing that Mason and Phil tricks Julien into believing that the "sky spirits" are angry with the Julien self-centrism, thing that change Julien's personality. But when Skipper notices that kind Julien is more annoying than the narcissist Julien, Skipper makes him come back to his original self.
It is also shown that Julien gains a super-lemur level of strength if he gets mad. Visible when in "Miracle on Ice", Julien spanked and humiliated the Rat King, after one of his henchmen touched Julien's feet; In Happy King Julien Day, Maurice claims that Julien violently used a cane to spank everyone who forgot his self-proclaimed holiday.
In Madagascar 3: Europe's Most Wanted, he has a bigger role and falls in love with Sonya the Bear. Also, he saved the day by managing to get back to the circus.