Buffy the Vampire Slayer
Dawn Summers (played by Michelle Trachtenberg) was a magical energy (The Key) that was turned into a person and Buffy's sister. At first, Dawn had the special ability to open the way to a hell dimension (with her blood). After Buffy's death closed that way, Dawn's only special abilities was being very good with languages and a very fast learner. She could have made a very good Watcher.