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Dumbo survived the destruction of his homeworld, but only as the Watergleam Gem, which wound up in Monstro where it was found by Sora. Sora took the gem to the Fairy Godmother in Traverse Town, and she activates the gem to allow Dumbo to assist Sora as a summon.
When summoned, Dumbo hovers in midair with Sora riding on his back. The two are invulnerable to harm until Dumbo is dismissed or runs out of MP. Dumbo blasts a jet of water from his trunk at the enemies, drenching them, pushing them backwards or defeating them. Dumbo can also ascend and descend in flight. Dumbo is a useful Summon and is best used when surrounded by Heartless.
When Sora summons Dumbo in Neverland, Dumbo is able to fly freely in the air instead of just hovering around. Sora shrinks to a smaller size to ride Dumbo during this summon.
In Kingdom Hearts: Chain of Memories, Dumbo appears as a Summon Card as a reward for completing Monstro. In the remake, Kingdom Hearts Re:Chain of Memories, summoning Dumbo causes Sora to leave the battlefield for the duration of the technique.
Dumbo is from Disney's animation film Dumbo. In the film, Dumbo is abandoned by his elephant kin due to his big ears but is later he is accompanied by Timothy mouse. With Timothy's help, Dumbo discovers the magic feather, that allows him to fly with his ears. This makes Dumbo famous and popular rather than a mockery.
 See also