Summon
A Summon is a special being that can be magically called on to aid Sora in the Kingdom Hearts series. They have been present in all released games so far, although the mechanics of using them differ slightly from game to game.
In the series, all summons have been Disney characters, with the exception of Cloud in Kingdom Hearts: Chain of Memories.
Game Differences
In the original Kingdom Hearts, they are obtained by finding special gems in the various Worlds of the game. The gems can then be given to the Fairy Godmother, staying at in Merlin's House in Traverse Town; she uses her magic to release their power and make the summon avaliable. All of these summons are victims of the Heartless' quest to destroy the Worlds. As explained by the Fairy Godmother, the summoning gems are the crystallized forms of strong, good-hearted denizens that did not fall victim to the darkness and destruction of their worlds, and so are able to be called into battle for a limited time.
In Kingdom Hearts II, Summons instead come from charms found scattered throughout the worlds, which Sora can immediately use without having to unlock them. It is hinted the charms of Kingdom Hearts II are different in idea and function from the gems of Kingdom Hearts; on one hand, the presence of Peter Pan as a summon drastically alters the idea of the summon gems from the original game, as Neverland was not destroyed but instead safely sealed. The charms instead appear to call the summon directly into to battle instead of giving their their lost soul a temporary body. At the same time, Mushu and Simba's worlds were restored, and so they are no longer available as summons since they can be found in their home Worlds.
Summon Mechanics
The Summon menu in Kingdom Hearts is located within the Magic menu during battle. But, in Kingdom Hearts II it was given its own area, the Command Menu, because of the inclusion of a second menu that could be accessed during battle. In addition, Kingdom Hearts II summons no longer require Magic to be summoned; they now use the Drive Gauge to count down their time instead.
