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.
The Return of Jafar
Abu and Aladdin enter a secret lair where they battle Abis Maland his gang and rescue a collection of jewels and a jeweled flower that they had attempted to steal. On the way back to Agrabah, they give the riches to poor folk in Agrabah and the jeweled flower to Jasmine, something Abu isn't too happy about but eases up when he realizes Aladdin and Jasmine's strong relationship. Abu and Aladdin go out the palace on a stroll and find Iago. Like Aladdin, Abu is very suspicious about the parrot and regrets taking him in even after he saves their lives from Abis Mal and his gang.
Later on after Genie returns from his world tour, Abu is given the job to guard Iago while Aladdin tries to explain his presence to Jasmine and the Sultan. While watching him, Iago pleads for freedom making rude monkey impersonations. Wanting to have a little fun with Iago, Abu opens the cage while Rajah is near as a way to scare him. However, the joke backfires as Rajah then destroys the cage and chases Iago into the palace, accidentally ruining the palace dinner and exposing Iago to the royals. Aladdin is ordered to watch Iago by the Sultan. That night, Aladdin and Jasmine settle their differences on Iago. The next morning Aladdin and the Sultan go for a ride on Carpet while Genie and Abu enjoy a picnic.
During the picnic, Abu spots a spider which soon leads to dozens of spiders. The spiders come together and form the revenge-seeking Jafar. Abu and Genie are tormented until finally being placed in captivity. The group is rescued by Iago and they set off to battle Jafar. At this time Iago is nearly killed and after defeating Jafar is thought to be dead. He proves himself alive and Abu finally accepts Iago as a friend.
Aladdin: the Series
Abu makes regular appearances in this television series, often paired with Iago. Although he is still a casual thief, he has shown definite signs of a conscience. Aladdin and Abu are shown to still have a strong friendship, but Aladdin sometimes becomes rather annoyed at Abu's thievery. Where on one occasion when Abu stole a golden gauntlet from the royal treasury and accidentally made Aladdin look like the thief, Aladdin became so mad that Abu was particularly heartbroken when he thought Aladdin wanted him to leave forever because of it. However, whatever troubles they are in, they always work it out and they become more closer than ever.
Through nearly all adventures, Abu is constantly paired up with Iago. Together, they often leave the mission to search and/or steal treasure from a nearby area. They are sometimes the cause of trouble. An example of this is in the episode "Smolder and Wiser" where Aladdin attempts to take a powerful weapon from Abis Mal, only to be discovered after Abu and Iago tried to steal a bag of gold, causing said gold to fall out, thus causing noise.
An important part of Abu's role is the fact that he has a pick that unlocks any lock which is handy when Aladdin and the group are being held captive in an area.
Aladdin and the King of Thieves
In the final installment of the Aladdin film series, Abu plays a fairly minor role compared to the previous films, providing very little to the plot. Like the rest of the characters, Abu is excited to finally see the day his best friend and Jasmine wed. Underneath all the joy, Aladdin is anxious and nervous being that he never had a father figure and Abu feels for his friend. When they discover his father is alive, Abu joins Aladdin, Carpet and Iago to see him. As soon as they arrive, Abu is captured and held hostage. In order to be freed, Aladdin must defeat one of the many thieves. He does successfully and meets his father Cassim.
Later on after the thieves betray Cassim, Abu joins the group in the rescue and after attends the wedding. He stays back at the palace with Genie, Carpet and the Sultan while Iago goes off adventuring with Cassim and Aladdin and Jasmine enjoy their honeymoon. According to Iago, Abu cried when the former bid his farewells to his friends in the palace, which in turn, made Iago emotional.
Aladdin (2019 film)
Abu appears in the movie. His role is largely unchanged.
Abu is a monkey that is obsessed with shiny items, due to his occupation as a thief. He is scheming and sneaky and hates danger. Abu is usually very kind, but can also be rather self-absorbed. Such as when 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.
The producers of the original film studied movements of Capuchin monkeys to use for the inspiration for Abu. Frank Welker also spent some time listening to them to get Abu's voice right.
Amusingly enough, Frank Welker would later voice another mischievous primate: Curious George.