^_^ Come to think of it : Strong hearts can turn into Heartless, and Xehanort's was the only one to have a humanoid form thus far. But that's because they willingly gave themselves up to the darkness and assumed the Heartless form, so you can very well imagine what amount of darkness they've got inside.
As for Sora, true that he willingly gave up his heart to save Kairi, but he had always fought against the Darkness. In a short span of time, do you think he would accept Darkness and become a Shadow just like that ? The element of unwillingness is there.
I'm as good as new! All my functions have been restored! • TroisNyxÉtienne