Abu is a supporting character in Aladdin. A mischievous thief with a love for all things valuable, Abu serves as the film's titular character's best friend, confidante and loyal partner-in-crime.
As mentioned above, Abu is Aladdin's pet monkey, who is obsessed with shiny items, due to his occupation as a thief, in addition to being a kleptomaniac.
Abu is usually very kind, but can also be quite selfish, feisty, and easily aggravated. In Disney's 1992 film Aladdin, Aladdin and Abu saw some hungry kids in the alley. Aladdin gave his share of the bread to the kids, and implored Abu to do the same. Abu at first considered himself down on his luck and hungry enough to be entitled to the bread, but soon found warmness in his heart and realized that the children were even worse off than Aladdin and himself. He is scheming and sneaky and hates danger. However, he is also very loyal to Aladdin and is willing to risk his life to save him and his friends. He tends to be Iago's partner in crime, but is less deceitful and more honest and trustworthy than Iago is. Like Iago, Abu can get frustrated rather quickly and hates to be made a fool of, especially by someone as close as Aladdin.
Abu acts as pet, sidekick, and best friend to Aladdin. Rather than being a genuine 'character', he is used as somebody for Aladdin to voice current thoughts, or sometimes to help him out of trouble - or sometimes to get him into trouble.
He makes his initial entrance while helping Aladdin escape from the Palace Guards after stealing bread. Unfortunately, he is attracted to a stall selling jewelery and has to be dragged away. Later in the bazaar, he helps Aladdin to steal a melon for breakfast, and when Aladdin meets Jasmine, he is almost caught picking a pocket. He also steals some apples. He doesn't really appreciate Aladdin's attraction to Jasmine, thinking it will spoil their friendship.
When Aladdin is imprisoned in the palace dungeons, Abu, who had not been captured, arrives and picks the locks of Aladdin's chains.
In the Cave of Wonders, Abu wakes up the Carpet. He is attracted to the many fantastic jewels and finally grabs a huge ruby out of a monkey idol's paws, initiating the destruction of the cave. When Jafar is about to stab Aladdin, he bites him on the arm, stealing the magic lamp in the process, but is thrown into the cave with Aladdin.
When Aladdin awakens the Genie, Abu is seen to be a tool for a mode of transportation, and goes through many changes before Genie decides on turning him into an Indian elephant (with a remarkably long monkey's tail). Abu is changed back into a monkey by Jafar, who then imprisons him, Aladdin and the Carpet into a tower and sends them to the ends of the earth.
After Aladdin, Abu and the Carpet returns to Agrabah to stop Jafar, Abu helps out by pinning and knocking out Iago, keeping him from alerting Jafar to Aladdin's return. Abu is transformed into a cymbal-banging monkey toy by Jafar when he tries to steal the lamp, but is restored once Jafar is imprisoned in his lamp after becoming a genie.