How different is Japan from America? From what I understand Marvel and DC don't seem to play by the same rules, as they seem to be unable to use characters that the other entity owns the IP to. What rights differences do authors have over their works in both countries? We all know about Nasu and how he seems to give away rights to his characters ad infinitum (Ciel and Higurashi, Prisma Ilya) but is this a quirk of the individual or is there an underlying principle in the law that doesn't grant him the power to exercise control over works that have been adapted from his?
Do doujin authors maintain full rights over their works even if they have been adapted (almost piece for piece) from existing series? What happens if the doujin material becomes more popular and generates increased revenue over the original? Does the author still maintain full rights and/or ability to stop the doujinshi?