She is part of a side quest then. You just haven't played it yet.
But that's still needless. It's a sidequest. I'd say no NPC should be essential that isn't part of the main quest line or the guild quest lines. Everyone else should be fair game. Murdered a woman? Oh, well, guess you won't get her sidequest.
And if nothing else those NPCs should really stop being essential AFTER you've done the sidequest. Because now if they die, who cares? It doesn't break anything.