I'm going to stick this here... Mercy has some natural immunity to magic. Vampires and the fae mostly can't bespell her (or she's able to break free of it), lots of magic just rolls right off, or it only affects her in odd minimal ways.

However, other magic works just fine. Canon says basically if it's relatively minor harmless stuff it doesn't affect her, but anything really nasty works pretty well.

She can also see ghosts and talk to them - if they're the kind that talk to you. Later on she'll also learn that she can control them, but she doesn't know that now.