Kingdom Hearts/Summons

Summons are characters that Sora can call mid-battle if he needs help and has enough MP.

Unlocking Summons
Summons can be obtained by collecting various Summon Gems across the worlds, but are only available after you visit Traverse Town a second time. After Sora is introduced to Merlin, examine the coach in Merlin's house (a reference to Cinderella) and the Fairy Godmother will appear. Speak to her and, if you have a Summon Gem, she'll transform it into a character you can summon. It will be added to your commands under Magic and then Summon. This doesn't apply for every summon, however. Genie and Tinker Bell can be obtained by completing their respective worlds - no Summon Gems are required.

How to Summon
After obtaining the Summon command, simply go to Magic and scroll to the bottom, where the Summon option can be found. After choosing that option, select the character you'd like to summon from the list of summons you found. You can only summon when you have enough MP (since it's a Magic command) and when both party members are present. You can't summon when Sora is fighting alone or when a party member is knocked out.

Using a Summon
After you have summoned who you want to, the chosen character will enter the battlefield in a short scene. A gauge will be in the top-right corner of the screen showing the summon's MP and Charge. Use the last option on the menu, Charge Up, to fill up this meter and allow your summon to attack. The MP bar represents the time the summon has left before it leaves and your party members return. It steadily goes down when the summon is doing nothing, and more MP is consumed when the summon attacks.

Simba
Simba is the first possible summon you can obtain at Sora's second visit to Traverse Town. After speaking with Leon, he'll give you Earthshine. Bring the gem to the Fairy Godmother and it will transform into Simba. When Simba is summoned, before entering the battlefield, he roars on Pride Rock and then launches into battle. As means of offense and defense, Simba uses his claws and fangs to continuously slash the enemies. When Sora is beginning to run low on HP, Simba cures him using herbs.

Genie
Genie, unlike Simba, is not obtained from a gem. The only way to obtain Genie as a summon is to seal Agrabah's keyhole. As part of the story, Aladdin suggests Genie accompany the party on their quest and give them a helping hand. When Genie is summoned, sparks will fly out of Sora's keyblade and Genie will appear on the battlefield. Unlike Simba, Genie doesn't require charging as he doesn't have a charge bar. All he has is an MP bar, which is consumed by using the Showtime command. He fights by using magic beams he shoots from his fingers and using the magic commands Sora has already unlocked. Genie will only attack if you are locked on to an enemy.