Zero

Zero is Jack Skellington's loyal pet ghost-dog from Kingdom Hearts, Kingdom Hearts II, and Kingdom Hearts 358/2 Days. He originally appeared in Tim Burton's The Nightmare Before Christmas. He also appears in Kingdom Hearts 358/2 Days alongside Jack, as well as getting a considerably expanded role in the game. However, in this game, Zero appears to be a light blue as compared to his usual white appearance.

Kingdom Hearts
''Jack's ghost dog. He comes out of his grave whenever Jack calls. He's very loyal to his master and loves it when Jack plays with him.''

He loyally followed Jack in "Tim Burton's The Nightmare Before Christmas" (1993).

Kingdom Hearts II
The Nightmare Before Christmas (1993)

''Jack's ghost dog. He comes out of his grave whenever Jack calls, and never gets tired of playing fetch with his master.''

Kingdom Hearts
Zero had a relatively small role in Kingdom Hearts, appearing only as a ghostly hallucination.

Kingdom Hearts 358/2 Days
When Roxas finds Jack and Zero in the Graveyard, he is amused to see that the ghostly dog can sense the Heartless's presence around him with his nose. Showing Jack a Carrier Ghost that was hiding in one, Zero is rewarded by a happy Jack while Roxas looks on. Roxas realizes that if he gives Zero a bone, the dog would reveal the Heartless' hiding places. Zero gladly complies, and Roxas completes his missions with ease.

Kingdom Hearts II
Sora encounters Zero again, aimlessly floating around Dr. Finkelstein's lab. When spoken to, he happily barks at Sora and his friends.

Appearance
Zero is a ghostly dog that appears to be made of a white sheet, much like a stereotypical ghost. Like Pluto, he has basset hound-esque floppy ears. His "eyes" are just empty holes in the sheet. Zero's snout is very long and thin, and his upper jaw curls upward at the tip. His nose is a small jack-o-lantern, and his collar is a faded brown color.

Origin
Zero first appeared in the 1993 movie The Nightmare Before Christmas. In the movie, Jack finds the door to Christmas Town while he is taking Zero for a walk. Later, when Jack wants to take off through the thick fog to deliver presents, Zero has to light the way. At the end of the movie, Jack and Sally kiss each other as Zero looks at them. He then flies into the air and transforms into a star.

Trivia

 * Unlike in past games, Zero's movement in Kingdom Hearts 358/2 Days is represented by the sound of blowing wind.
 * When you run up to Zero in Kingdom Hearts, he would disappear and you would go right through him, while he was able to interact with Sora in Kingdom Hearts II.

Zéro Zero