Okay here goes... (I'm guessing we can take stuff from KH1 and KH2)

Kairi, Princess of Heart
The reasons why I chose Kairi.

Reason 1:
She is the heroine in both games. In KH1 she is one of the seven Princesses of Heart and is sought after by Maleficent and the other Bad Guys.

Reason 2:
In KH1 she turns Heartless Sora into real Sora again. She hugs him and is able to give him back his heart.

Reason 3:
In KH2 when Riku gives her a Keyblade she wields it and uses it to defend Sora. This shows how she has been able to help him at least twice and without her the story would never have been able to continue.

Quite short I know.