Bambi is a character from Kingdom Hearts and Kingdom Hearts: Chain of Memories.
Bambi's world was destroyed by the Heartless. However, the young fawn's will allowed it to be preserved in the Naturestone item. After Sora obtains the item in the Hundred Acre Wood and takes it to the Fairy Godmother, Sora gains the ability to summon Bambi in battle.
Bambi's role in battle is to prance about the area, dropping huge amounts of HP Orbs, MP Orbs, Munny Orbs, Items, etc. Any enemy defeated while Bambi is summoned drops more orbs than usual. Also, the more enemies Sora defeats while Bambi is summoned, the more orbs Bambi drops.
In Kingdom Hearts 2, Bambi's role is essentially taken by Peter Pan. Bambi's world has yet to appear in any game.