Over the course of time, publishers go out of business, creators die or whatnot. After a while, if their copyrights are not acted upon, their characters will fall into the public domain, free for anybody to use (as derived from their original source, not from any others' depictions.
This category will also include open source characters, which are created fo anybody to use, but usually with a consideration for their use (often to perpetuate that they are, in fact, free to use and nobody can claim copyright).
Feel free to go to the public domain superheroes wiki for more information.
Pages in category "Public domain characters"
This category contains only the following page.