Hsi Wu was the Demon Sorcerer of Sky who hailed from a family consisting of himself as well as seven siblings and later on his nephew Drago.
Since he's the Sky Dragon, he appears to be a human-sized black-skinned demon with wings.
He was one of the last Demons to be released, his portal located outside the women's room in Fenway Park. Jade accidentally snapped off his tail, forcing him to flee. He tried to retrieve his tail but Uncle placed a spell that would prevent demons from entering the shop unless they were invited. Seeing Jade, Hsi Wu saw his opportunity. He disguised himself as a human boy named Seymour Jahoositz and quickly befriends Jade. He gets inside the shop and nearly to his tail only to be pushed out by Uncle. Next day, he was invited to the Spring dance by Jade and gave him a necklace with one half a coin. He once again tries to get into the shop only to find that Uncle had hexed his tail to immobilize him. Hsi Wu catches his tail in a bag, revealing himself to everyone (including to the horror of Jade). He kidnaps uncle, demanding that he undo the spell; Uncle said that he cannot and required to do another spell, but needed books. The Dark Hand brought spell books but their efforts were hampered by Jackie and Tohru. Hsi Wu flew off with his tail, determined to find some other way. But Jade, empowered by the Rabbit and Rooster Talismans, begins a flight chase across the industrial district. She is able to release his tail on him, as Jackie played the flute, sending him back to the Netherworld.
Later, after Bai Tza becomes the seventh demon to be banished, the demons hold a meetings to discuss Shendu's punishment. In desperation, Shendu claims that his portals can let them all out and that he had discovered a spell to keep it open until all of them were freed. While waiting for Shendu to open the portal, Hsu Wu is hit by a Moose camera. He followed its path to its source, Jade, who got caught in the banishing spell's beam and was pulled into the Netherworld after Bai Tza. He brings Jade to his siblings, which pleases Po Kong with the prospect of her first meal. But Jade reveals that, contrary to Shendu's story, only one being can go through Shendu's portal, causing debate amongst the Demons. When the portal opens via the banishment of Shendu's spirit, it becomes "every demon for himself." Hsi Wu is first off the bat, but he is stunned with lightning in the back from Tchang Zu. However, he pulls back ahead by knocking down Dai Gui and Tso Lan with boulders. Seeing all the other demons out of the running, he looks ahead only to see Jade jump through the portal, which closes in his face, leaving his last burst of speed to send him plowing into a rock.
Enraged at Shendu's deception, the demons prepare to torture him for all eternity. In desperation, he suggests using the Book of Ages, which horrifies the others, for they would never tamper with that. But with their current situation, they reluctantly agree. Rewriting the book, Shendu makes it that he and his siblings were never banished, making them rulers of the world. Hsi Wu has his own tower high in the sky, with Viper as his songbird. While scrubbing for Tchang Zu's arrival, he hears a flute, which he despises. It turns out to be played by Jackie, whose memory was restored, and Hsi Wu is once again banished.