Off the top of my head: most of the central cast of Star Trek, Irene Adler from Sherlock Holmes, at least two people from the Batman franchise, Edward Sexby but not strictly the historical version, the Marquis de Sade (again, heavily reimagined), Snape, Draco and Grindelwald from the Harry Potter books, O'Brien from 1984, Elizabeth Bennet, Princess Leia...I could go on.

So, common? Extremely. I think the rule is that for everyone here, there is at least someone who'll consider them fictional.