Pluto

Pluto is Mickey's loyal pet dog and also appears to be a messenger for him. Pluto is one of the few characters to have a proper voice actor, but his barking is portrayed by Bill Farmer, who also voices Goofy. Pluto debuted in the 1930's animated cartoon, The Chain Gang. Pluto has so far appeared in all games released so far, but he has continous absence throughout and appearing unexpectedly during certain points and no explaination of his absence has been given.

Kingdom Hearts
''Mickey's faithful dog. Pluto is more than a pet. He and Mickey are bound by strong ties of loyalty. Pluto set out with the others to find his master. Will that famous nose of his lead him to Mickey? Pluto originally appeared in "The Chain Gang" (1930).''

Kingdom Hearts II
''The King's faithful dog. But Pluto's more than a pet. He and King Mickey are bound together by the strongest ties there are: friendship and loyalty. When the King disappeared, Pluto set out with the others to find his master.''

I hope that famous nose of his will lead him to the King.

Kingdom Hearts
Pluto has a minor role in Kingdom Hearts but a notable one. Pluto is found by Donald in the throne room of Disney Castle with a letter from the King who has gone missing (he has actually gone off to fight the Heartless). Pluto later sleeps during the conversation of options between Queen Minnie, Donald, Goofy, Daisy Duck and Jiminy Cricket. Donald, Goofy and Jiminy are sent to locate the King and the Keyblade Master. Pluto hops into the Gummi Ship at the last moment as it departs.

In Traverse Town, the group witness the disappearance of a star and rush off to find Leon. Pluto wanders off into an alley way, discovering an unconscious Sora, who the dog wakes up with a lick and a sudden shove. Sora asks Pluto where they are, but the dog seems to hear something and rushes off. He disappears for the remainder of the game, although it's possible he was reunited with King Mickey - as he appears suddenly at the end-credits scene carrying a letter with the King's seal. Sora, Donald and Goofy chase him over the hills.

Kingdom Hearts: Chain of Memories
Pluto has a brief appearance in Kingdom Hearts: Chain of Memories at the beginning. Sora, Donald and Goofy spot that he is carrying an envelope with the King's seal. Pluto rushes off, the three running after him. However, it appears they lost track of him or gave up due to it being night. However, the pursuit of Pluto leads the trio to a crossroad which eventually leads to Castle Oblivion, and the events of the game. It is possible this Pluto is actually a memory form created by Naminé to lure Sora to the castle.

Pluto appears as a friend card in Re:Chain of Memories. He digs up health and moogle points for Sora when summoned. It is unknown if this is the real Pluto.

Kingdom Hearts II
Pluto returns in Kingdom Hearts II, oddly in The World That Never Was for unknown reasons, although he seems to be tracking Axel. He chases Axel into a dark corridor portal, but a hidden Riku opens another for him. Pluto arrives on Destiny Islands just in time to prevent Kairi from going with Axel. Pluto and Kairi escape into Twilight Town where they meet Hayner, Pence and Olette. Axel soon appears and kidnaps Kairi, Pluto chasing after them.

Pluto and Kairi end up imprisoned in a cell in the Castle That Never Was. They are rescued by Naminé and escape. They soon confronted by Saïx, but Riku appears in his disguise as Xehanort's Heartless and chases off Saïx. As Riku is about to leave, Pluto blocks his path and moves Riku in the direction of Kairi. Riku allows Kairi to remove his hood. Pluto disappears until the defeat of Xemnas, leaping into a portal made by Naminé which leads to Destiny Islands. Despite the fact that Kairi, Donald, Goofy, King Mickey and Pluto passed through the portal to the islands, Pluto was not present when Sora and Riku returned. Pluto is seen in the credits with Donald, Goofy and King Mickey when they return to Disney Castle.

Kingdom Hearts coded
Pluto will be reappearing in Kingdom Hearts coded. Screenshots have shown Pluto and Sora in an alley in Traverse Town.