Then what are "good" main characters in your view? I don't really understand what the problem is.
That would also always depend on how one defines "main character", I noticed that many people tend to mix up the terms "major character", "main character" and "protagonist".
major characters are people who have recurring appearances and almost always contribute importantly to the plot in some way, but get less focus on their own background.
In KH I would put characters like Donald, Goofy, Axel, Saix, Xigbar, Yen Sid, Ansem the Wise, Maleficent, Vanitas (as of now) or Eraqus in that category.
main characters are people who appear even more often than majors, are closely involved with the plot, are playable in some instances and get also more background information.
In that category I would put Kairi, Xion, Namin?, Mickey, Terra, Aqua, Ventus, Xehanort, Ansem SoD, Xemnas and Roxas.
protagonists are of course the characters with the most importance who are playable most of the time, get the most attention and background information.
For KH this would be Sora and Riku obviously.