Pluto is King Mickey's pet dog in Kingdom Hearts. He belongs to Disney Castle's royal household and is closely associated with Mickey, just as he is in Disney's traditional animated shorts.
In the first Kingdom Hearts, Pluto helps connect the game's opening events to its larger mystery. After Mickey leaves Disney Castle to investigate the spreading darkness, Pluto appears carrying a letter from the king. Donald and Goofy follow him, eventually setting out to find the Keyblade wielder and their missing ruler.
Pluto is often confused with Goofy because both are anthropomorphic canine Disney characters. Their roles are very different: Goofy is a talking royal guard captain and Sora's traveling companion, while Pluto is Mickey's pet and generally communicates through barking and physical reactions. Pluto's Disney history also predates Kingdom Hearts: he first appeared in 1930's The Chain Gang and became Mickey's named pet soon afterward.