In each game you get to summon friends to help you besides Donald and Goofy, but they can only help you for a little bit. In the first game when you use a summon character you use up magic. In the second game when you use summon characters you use up your drive bar.
These are th summon characters in the game Kingdom Hearts I:
After a wish freed him, Genie told Sora to give a call if he needs help. When you summon Genie he floats behind Sora. When you use the "Show Time" comand he goes crazy casting spells and curing Sora if needed.
Tinkerbell becomes a summon after the clock tower is fixed. When summoned she uses Cure almost every time Sora loses health.
Mushu become a summon after Malifecent is defeated. He appears on Sora's head and blows fireballs.