Bat-Mite is one very curious DC Comics character. For is officially a canon character, but can only be used sparringly as he's from the Fifth Dimension, a place where he "knows" what Batman is and isn't (as in real and in a comic book/show/movie/etc.
His biggest role was in the Batman: The Brave and the Bold series, where he appeared as Batman's #1 fan, and often sought to "help" him, even when Batman said he didn't need it.
In fact, Batman has often had to stop Bat-Mite from taking over his world in unintenional ways. And it was Bat-Mite that "caused" the cancellation of Brave and the Bold, which ironically "cancelled" his existence.