A Summon is a special character that can be magically called on to aid Sora in the Kingdom Hearts series. They have been present in Kingdom Hearts, Kingdom Hearts: Chain of Memories, and Kingdom Hearts II although the mechanics of using them differ from game to game.
Game differences
Kingdom Hearts
In the original Kingdom Hearts, most of the summons are obtained by finding special gems called "Summon Gems" in various worlds. The gems can then be given to the Fairy Godmother, staying at Merlin's House in Traverse Town; she uses her magic to restore their spirit forms and release their powers, thus making the summon creatures available. All of these summons are victims of the Heartless' quest to destroy the worlds. As explained by the Fairy Godmother, the summon gems are the crystallized forms of strong, good-hearted denizens that did not fall victim to the darkness during the destruction of their worlds, and so are able to be called into battle for a limited time.
Two summons, Genie and Tinker Bell, are allies from worlds Sora has visited, and join him of their own free will upon locking the Keyhole of their world. They do not need to be released by the Fairy Godmother before they can be used.
Kingdom Hearts: Chain of Memories
In Kingdom Hearts: Chain of Memories there are Summon Cards that Sora obtains during his ascent of Castle Oblivion. Some Magic Cards enable Sora to perform summons. These cards are created from Sora's memories, so the summons are the same ones from Kingdom Hearts, with the addition of Cloud. They also differ in how they play out, as playing or stacking a different number of cards has different results, much like Magic and Friend cards (for example, one Cloud card will use a normal slash, while two make a Cross-slash and three make an Omnislash). However, Sora is unable to activate cards while the summon is active, contrary to Enemy Cards.
Kingdom Hearts II
In Kingdom Hearts II, summons come from special charms found scattered throughout the worlds, which Sora can immediately use without having to unlock them. The charms from Kingdom Hearts II work differently than the summon gems from Kingdom Hearts. While most of the summon spells on the first game gave a temporary body to the soul contained within the gem, the charm actually teleports the summonable character directly from their world.
Kingdom Hearts Birth by Sleep
In Kingdom Hearts Birth by Sleep, a new form of summon featured, known as the Dimension Link (shortened "D-Link"). Players can draw power from both non-playable characters or summon one of the three main characters wirelessly from other players to aid them in action. While Cinderella, King Mickey, Zack Fair, Experiment 626, Donald Duck, and others are known as the non-playable D-Link characters, Terra, Ventus, and Aqua are the wireless D-Link characters.
Mechanics
Kingdom Hearts
The Summon menu in Kingdom Hearts is located within the Magic menu during battle. Each summon requires a specific amount of MP to be performed, and the summon lasts until its MP bar is fully consumed (or until the player dismissed it manually). In order to perform a summon, the player needs to have two friends in the battle with him, and they both must be conscious. This means that Sora cannot perform summons during battles he is fighting if he is alone or when one or both of his allies are knocked unconscious.
Kingdom Hearts: Chain of Memories
In Chain of Memories, once Sora plays that specific summon's card, the summoned character will appear, perform one or more attacks/actions (depending on the amount of cards stocked) and immediately disappear.
Kingdom Hearts II
The Summon menu was given its own area in the Command Menu in Kingdom Hearts II because of the inclusion of a second menu that could be accessed during battle. In addition, Kingdom Hearts II summons no longer require MP to be summoned; they now use the Drive Gauge to count down their time instead, and again the player must have two friends in battle in order to perform a summon.
Leveling Up Summons
Summons in Kingdom Hearts II draw their power from a separate Summon Gauge (similar to the Drive Gauge), which can also be leveled up, allowing the Summon to remain with Sora for a longer period of time. One point of Experience is gained for every gauge expended while the Summon is on the field. A simple way to level up is by finding a safe place, call a Summon, and then wait as time whittles down the gauges before jumping into the Gummi Ship to reset the gauge to MAX.
However, a much faster way to level up the Summon Gauge is to call a Summon to the field, and then find enemies to fight. Closely watch the Summon Gauge, and attack enemies the moment the Gauge drops down a level. Each time you land an attack, the gauge will be boosted a little bit, so by constantly attacking to boost the gauge back up a level, it will drop back down again. Continue this method to boost the gauge back up a level each time you attack so it is then expended, and you'll acquire a point of Experience each time.
List of Summons
Kingdom Hearts
Simba
From the Earthshine Gem obtained from Leon during the second visit to Traverse Town.
Bambi
From the Naturespark Gem obtained after returning the first set of pages to Pooh's book.
Dumbo
From the Watergleam Gem found inside Monstro.
Tinker Bell
She joins the party after sealing Neverland's Keyhole.
Mushu
From the Fireglow Gem obtained after defeating Maleficent in her Dragon form.
Kingdom Hearts: Chain of Memories
Simba
Leon gives Sora the Simba Card after he completes his tutorial.
Genie
Genie gives Sora the Genie Card after he completes the Agrabah floor.
Bambi
Sora obtains the Bambi Card after completing the 100 Acre Wood floor.
Dumbo
Sora obtains the Dumbo Card after he completes the Monstro floor.
Tinker Bell
Tinker Bell gives Sora her Tinker Bell Card after he completes the Neverland floor.
Mushu
The Mushu Card is located in a chest in the Key to Rewards Room on the Hollow Bastion floor.
Cloud
Cloud gives Sora the Cloud Card after he completes the Olympus Coliseum floor.
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Kingdom Hearts II
Chicken Little
Merlin gives Sora and co. the Baseball Charm after he tells them about Pooh's book.- Genie (Valor).png
Genie
Genie gives Sora the Lamp Charm after his first visit to Agrabah. 
Stitch
The Ukulele Charm is placed in a chest in Ansem's Study during the second visit to Hollow Bastion. Its chest only appears after Sora and the others return from Space Paranoids to find the password to the dataspace.
Peter Pan
The Feather Charm is in a chest in the Ship Graveyard during the second visit to Port Royal.
Trivia
- Peter Pan, Genie, Simba, Stitch, and Cloud are the only characters to appear as both Summons and Party Members.
- Mushu and Simba are summons in the first game, but become world-specific party characters in Kingdom Hearts II.
- The ultimate Summon from the Final Fantasy video game series, Bahamut, was intended to be included in Kingdom Hearts, but was cut before the final release. Through hacking, it is possible to gain a "Bahamut" option in the Summon menu; however, it is an incomplete summon, and therefore will freeze the game if selected.
- The main purpose of the Summons were to add in any extra Disney movies that weren't already represented by a World.
- Kingdom Hearts 358/2 Days is the first game in the series not to incorporate any form of summons.
- Dumbo, Bambi, and Chicken Little are the only summons whose worlds haven't been present in any games yet.