Cassim is a major character and Aladdin's father in Aladdin and the King of Thieves.
Cassim is the father of Aladdin. Long before the events of the the first film, he left his wife and young son in order to find a better life for the poverty-stricken family. While he was gone, Aladdin's mother died, leaving Aladdin as a presumed orphan. As such, when Cassim returned to Agrabah, he could not find his family and forcibly returned to the life of thievery. At some point, he became involved with the Forty Thieves, eventually becoming their leader, and began a search for the Hand of Midas, which could turn things into gold.
In the third movie, Cassim leads a raid on Agrabah in an attempt to find the Oracle, which could answer a single question about any subject, in the hopes of using it to find the Hand of Midas. The raid is ultimately unsuccessful but interrupts and ruins Aladdin and Princess Jasmine's wedding. He gets in a fight with the groom, Aladdin, unaware that he is his son as he has not seen him since he was a baby.
After the wedding, Aladdin unexpectedly finds the Oracle, and learns of the survival of his father, whom he had presumed was deceased, and his location with the Forty Thieves. Aladdin tracks the Forty Thieves to their hideout but learns that his father is actually Cassim, their leader, rather than a prisoner. Cassim is overjoyed to be reunited with his son after so many years of thinking he was dead but is interrupted by Saluk, saying Aladdin must die because he intruded on their hideout. Cassim, knowing he has no choice, seemingly agrees, then suggests Aladdin face "The Challenge", which will grant him membership in the Forty Thieves if he succeeds.
Aladdin gains entrance into the Forty Thieves by fighting and defeating Sa'Luk in a fight. After learning from Cassim the real reasons for his departure from their family, Aladdin decides to help Cassim get into the palace. At first, things go very well and he quickly bonds with his future daughter-in-law Jasmine and the Sultan. However, Cassim is convinced by Iago to rob the Oracle in the royal treasury and is arrested and sentenced to life imprisonment. Aladdin helps Cassim escape prison, but refuses Cassim's pleas to flee the city and abandon Jasmine as his father did with him. Aladdin heads back to Agrabah to take responsibility for his actions, while a reluctant Iago joins Cassim alone.
Cassim returns to the Forty Thieves with Iago, bitter from his fight with Aladdin, saying the thieves are his only family, where he discovers that Sa'Luk has usurped control and turned the remaining Thieves against him. Cassim is then imprisoned by the Thieves. In order to prove his loyalty to Sa'Luk and the gang, Cassim uses the stolen Oracle to locate the Hand of Midas and leads the Thieves to its location, a marble fortress on the back of a gigantic sea turtle.
Aladdin eventually returns, having been warned by Iago of his father's capture, and releases him. The two reconcile and recover the Hand of Midas. Suddenly, Sa'luk attacks and threatens Cassim to give the Hand to him, or else he will kill Aladdin. Refusing to sacrifice his son for the treasure he sought, Cassim tosses the Hand to Sa'luk, who catches it by the golden hand itself and is instantly transformed into a permanent golden statue and falls into the sea below. The more athletic Aladdin jumps to recover the Hand, then returns it to Cassim, remarking to his father that it pays to have a junior partner.
After escaping, Cassim has a change of heart and chooses to throw the Hand away, having realized the pain his obsession brought and that Aladdin was more precious to him the whole time. Although Cassim did not seek revenge against the Forty Thieves for their betrayal, he unintentionally causes their demises as the Hand accidentally lands on their ship, turning it to gold. The solid gold ship proves unseaworthy, and it sinks to the bottom of the sea.
Cassim returns to Agrabah with Aladdin, and witnesses Aladdin's wedding from the shadows, as he is still a wanted criminal. Cassim then accepts Iago as a traveling partner, and the pair travel to see the world, but not before waving goodbye to his son and new daughter-in-law who were flying on their Magic Carpet.
In The Legacy of Chris Thorndyke he Is still out there somewhere but he is not just a thief but an assassin and a good one too and leads the Neo 40 Thieves Which he is still working on a name for the band of Thieves and did Aladdin to join, but Aladdin turned it down, but still remains an ally. He also had some tabs on a certain Legendary assassin with a white Robe and lack of a ring finger on his right hand and he even had a similar hidden blade but at least he kept his finger.
And he is reprised by John Rhys-Davies in the English dub and Masaru Ikeda in the Japanese dub. (But in some point in time, that someone might voice him if it is a tribute.)