There are two characters. Maybe one is never seen, or is only mentioned in passing, or whose face is never seen, or it's a book and the author ain't much for descriptions. These two never meet, are never in the same scene, and/or never interact with the same characters. Eventually it's revealed that they're actually the same person. Note that, contrary to what the trope name suggests, they do not actually need two aliases (i.e. pseudonyms) for this trope to apply - but either the characters in-universe or the reader/viewer (or both) need to believe them to be separate people, until The Reveal.