フェアユース
Fair use is assessed on a case-by-case basis. While there are no bright-line rules, such genres as parody and criticism are enumerated by statute and case law as presumptively fair uses. There has been no case law that squarely addresses fanfiction in relation to fair use. Works of fanfiction are more likely to constitute fair use if they are "transformative" with respect to the original work, if they are non-commercial, if they appropriate relatively little of the original work, and/or if they do not tend to detract from the potential market for or value of the original work.