The following is not legal advice and should not be relied upon to take or refrain from taking any action and does not apply in particular to any of the publishers identified in the question.
A fictional character can receive copyright protection apart from the work in which it appears if the character is presented with "consistent, widely identifiable traits." A good article on the subject is found here: . It is unlawful, therefore, under those circumstances, to create and sell books containing characters first created by someone else without a license to do so.
The "use" of another's copyright protected character, however, may be a "fair use" depending on how and how often the character is "used." That analysis is very fact specific. A website that provides information about fair use can be found here: . Adding a disclaimer such as "Unofficial" or Unauthorized" to the subsequently created work provides NO protection to an infringer under copyright law.
If you are considering creating works that contain fictional characters first created by someone else you need to consult with an experienced copyright attorney.