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Zero is Jack Skellington's loyal pet ghost dog. He made his Kingdom Hearts series debut in Kingdom Hearts; he later appeared in Kingdom Hearts II and Kingdom Hearts 358/2 Days. Zero originates from Tim Burton's The Nightmare Before Christmas.
When Lock, Shock, and Barrel steal Dr. Finkelstein's artificial heart for Oogie Boogie, Jack Skellington sends Zero to follow them. Jack, in turn, teams up with Sora, Donald, and Goofy and follows Zero to Oogie's Manor.
Kingdom Hearts 358/2 Days
When Roxas finds Jack and Zero in the Graveyard during Mission 45, he is amused to see that the ghostly dog can sense nearby Heartless with his nose. Zero reveals a hidden Carrier Ghost to Jack, who happily rewards him. Roxas learns by watching this that if he gives Zero a bone, the dog will reveal other concealed Heartless. Using this tactic to his advantage, Roxas completes his mission in Halloween Town with ease. Later, Roxas repeats this tactic to complete Mission 52.
Kingdom Hearts II
Sora encounters Zero again, aimlessly floating around Dr. Finkelstein's lab. When spoken to, Zero happily barks at Sora and his friends.
Zero is a ghostly dog who closely resembles a white sheet, much like a stereotypical ghost. However, in Kingdom Hearts 358/2 Days, Zero appears to be a light blue in coloration, contrasting his usual, white appearance. Like Pluto, Zero has basset hound-esque, floppy ears. His "eyes" are just empty holes and his snout is very long and thin. Zero's upper jaw curls upward at its tip, and his nose is tipped with a small jack-o'-lantern. Zero's collar is a faded brown color.
Zero first appeared in Tim Burton's 1993 film The Nightmare Before Christmas. In the movie, Zero spends a majority of his time at the side of his owner, Jack Skellington. Zero is present when Jack finds the door to Christmas Town, and due to his glowing nose, he also becomes the leader of Jack's team of skeletal reindeer on Christmas Eve when Sally tries to use a thick fog to stop Jack from taking off to deliver presents. At the film's conclusion, as Jack and Sally kiss atop Curly Hill, Zero flies off into the night sky and transforms into a star.