OCs (original characters) are not evil, but you have to know when to use them.
1. If you want a character with a certain personality (note: When I mean a personality, I don't mean "perfect".) and don't want to make a character OOC (out of character), use an OC.
2. If you want a character from before or after the Kingdom Hearts series, go ahead and use an OC.
3. If you want to make an original world (like, for example, a world that takes place in, say, the Roman Empire or even an entirely original planet), then use an OC.
4. If you want to make a fic that is outside of the events of Kingdom Hearts but takes place in the same timeline (such as an original plan by the organization that focuses on, say, KH2!Hollow Bastion), then that provides a good oppritunity to use an OC.
5. You can even make semi-OCs, using characters that are mentioned or implied but are considered extras, such as Sora's mom, the judge for the Struggle contest, and (featured in my next plotbunny list) the maker of Sea Salt Ice Cream. These characters can be developed more fully, with their own personalities.
There are plenty of reasons to use an OC, but there is no reason you should make the character flawless or based on an idealization version of yourself (which is fine to do privatly, but you may want to expect ridicule if you make the fic public).
P.S - I made my first fanfiction! It's called "The Chair".
http://www.fanfiction.net/s/3606085/1/P.S.S - Where can I buy Sea Salt Ice Cream?