Kingdom Hearts, The Journey Begins
13 parts Complete Life on the Destiny Islands was fun for friends Sora and Riku and twins Kairi and Kate, when darkness attacks their home, everyone finds themselves separated from one another, friends Sora and Kate must team up with Donald and Goofy to not only find Riku and Kairi, but to save the other worlds as well before they all fall into darkness.
(This story has been moved from a writing website I was previously on, and re-written)