Julius is a giant hulking Frankenstein-like version of Pete that was created by Doctor Frankenollie. A monster (of fairly low intelligence) who hosts Mickey's brain for a short time. He has been identified by some as Peg Leg Pete, because of the physical similarities and the missing leg on his left foot.
Julius is unleashed from his lair under Frankenollie's lab to switch brains with Mickey Mouse. The experiment was done successfully although it killed Frankenollie in the process. However, Julius and Mickey not only switched brains, but bodies as well. In Mickey's body, Julius was able to view Mickey's wallet where he finds a picture of Mickey and Minnie. He develops feelings for her and rushes out to find her. He spots her at a shopping store and enters to proclaim his love. Unfortunately, Minnie thinks it's Mickey she's talking to and when the real Mickey in Julius' body enters to save her, she attacks him. Mickey is able to explain and battles Julius while Minnie is safe on top of a skyscraper. Mickey and Julius end up being electrocuted on cable wires, switching back in the process. Now that Julius is in his rightful body, he has a better chance of destroying Mickey and getting Minnie. Still, Mickey manages to free Minnie from his grasp. He is accidentally knocked off the building by Minnie but is pulled up repeatedly like a yo-yo as he is tied to a rope attached to a moving Hawaiian girl's arm on a billboard. In the end, Mickey and Minnie use Julius to pull a raft to Hawaii for vacation.
Julius appears as a secret boss in the game, located in Traverse Town. He emerges from a sewer grate below to the surface to attack Sora or Riku. After defeating him, he retreats back to the sewer grate in a sudden panic and Sora or Riku earn the Ultima Weapon Keyblade. He's the first secret boss in the Kingdom Hearts series to be a Disney character that is not exclusive to the series. Why he attacked the boys or how he wounded up in Traverse Town is unknown.
As with other past secret bosses having some importance in the story of the next game (such as Sephiroth being part of the Kingdom Hearts II story or Young Xehanort from Dream Drop Distance), its unknown if Julius will play a role in the next game.