Writing a new story using characters someone else has developed is theft. Your relying on the history of such characters as developed by the original author to give basis to your story.

Just naming someone Luke Skywalker and writing a story about a drunk cabbie with that name would not be derivative and not likely a problem, writing a story about a Luke Skywalker battling evil on some god forsaken world would be because you know his history, the new author had someone do all the hard work for him - making an appealing character, making a character with intrinsic market value.