Zero

Zero is Jack Skellington's pet ghost dog. He originally appeared in Tim Burton's "The Nightmare Before Christmas". He shows up in Kingdom Hearts and Kingdom Hearts II at Halloween Town to help Sora and company. He will appear in Kingdom Hearts 358/2 Days alongside Jack.

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.''