The Uncanny Isabella, or simply Isabella, is a human man.
Isabella was a traveling performer, describing himself as a "Master Magician". His shows mostly relied on tricks and prestidigitation, though he was also not against shopping of true magical items to improve his performances. A short time after his coincidental lookalike Jafar's first defeat and exile to the Cave of Wonders, Isabella came to perform at the Palace of Agrabah, where he enthused the Sultan but only made Aladdin and Jasmine long for "the real thing", namely their friend the Genie, whose absence was sorely felt as he travelled the world.
Shortly after this performance, Isabella, visiting Agrabah's bazaar in the hope of finding some new mystical item, was conned into buying Jafar's Lamp from a disguised Iago, and let himself be manipulated by a mix of threats and promises to help Jafar abduct the Sultan from the Palace and get his revenge. As the malevolent genie began to unveil the full extent of his cruelty, however, Isabella was convinced of the error of his ways and used his third and last wish to banish Jafar and Iago back to the Cave from which they had escaped. The good Genie promised to teach him some real magic in compensation for the sacrifice of his last wish.
Behind the scenes
Isabella appears in the 1993 story More Arabian Nights.
Although his resemblance to Jafar goes uncommented-upon, the later revelation in A Whole New World: A Twisted Tale that Jafar was sold into slavery as a young child and does not remember so much as the names of his parents suggests that Isabella could well be a relative (or even sibling) of the ambitious Grand Vizier and sorcerer, without either of them knowing it.