Shendu is the Demon Sorcerer of Fire in the form of a humanoid dragon. Shendu hailed from a family consisting of himself as well as seven siblings and his own son, Drago. Although Jackie Chan Adventures has no sole underlying main villain for the whole series, Shendu is arguably the most prominent villain and best candidate for the role of a main antagonist, due to his role in the events of the first and second seasons, his revival at the end of the third and the rise of Drago in the fifth, and the occurrence of the events in the fourth season owed in part to the events of the previous seasons and his use of one of the Shadowkhan tribes.
It's unknown how or when Shendu came into existence, but he and his seven Demon Sorcerer siblings terrorized the Earth during ancient times; until they were all defeated and banished to the Netherworld by the Eight Immortals. Shendu was specifically banished by Lu Dongbin, and his portal was located in what would later become modern Hong Kong, indicating he was already present there before Lo Pei's rebellion.
It's implied Shendu and his siblings had some history with the Oni of Japan, as their Demon Archive possessed Tarakudo's magic mark on the cover, and Shendu had one of the Oni Masks imprisoning Tarakudo's generals stored in his palace. Among Shendu's other magical accomplishments, it's suggested his Noble Animals-derived powers originally belonged to actual animals before he possessed them.
Nine hundred years prior to the show's main time period, Shendu was on Earth and ruled a vast empire from in Ancient China; indicating he'd somehow escaped the Netherworld after his banishment. His siblings, meanwhile, all remained imprisoned and Shendu made no attempt to free them; which would earn him their eternal bitterness centuries later.
Shendu's reign ended when his subjects rebelled with Lo Pei, casting a spell which turned Shendu into an immobile statue, while extracting his twelve Noble Animal-derived powers and turning them into Talismans, which were scattered around the world. Shendu vowed revenge for his downfall by destroying all of Asia - a task which he was unable to accomplish in his statue form, forcing him to spend the next several centuries looking to reacquire all his missing Talismans so their power would restore him.
It was revealed in Showdown in the Old West that Shendu was in an old west town during the nineteenth century. He attempted to acquire his Horse Talisman, allegedly among several antiquities being transported through the town to a San Francisco museum, with the aid of the Mayor, but his minions failed.
And he is reprised by James Sie in the English dub and he will be voiced by Akio Otsuka in the Japanese dub.