Jade

Jade is the niece of Jafar and cousin of Jay. She's briefly mentioned in the novel Isle of the Lost and Descendants 2: Uma's Wicked Book (For Villain Kids).

Isle of the Lost
Jade is mentioned by her cousin in the novel once. Her 16th birthday party is mentioned along with a handful of other events Mal didn't attend.

Descendants 2: Uma's Wicked Book (For Villain Kids)
Jade is briefly mentioned among those who have wrestled crocodiles. Jade attempted to wrestle a six-foot crocodile underwater. This proved to be a mistake because she almost drowned.

Trivia

 * Jade is mentioned once in the novel during Chapter 6, Mean Girl.
 * It is yet to be confirmed who is Jade's mother. It's speculated that Nasira, Jafar's sister in Aladdin in Nasira's Revenge, is Jade's mother, but since Descendants does not consider Disney sequels, movies or TV series, canon, Nasira cannot possibly exist within the franchise.
 * It is also currently unknown whether she is related to Jay through Jafar or Jay's mother. though she is likely through Jay's father and that her mother is Nasira as we have no idea who Jay's mother is as he states that he doesn't know his mother but Is familer with his cousin
 * It is unknown if she even joined Yen Sid's Anti-Heroes Club.