Most tragic character?
Or just the character you felt most sorry for? Merlin? Mab? Frick? Morgana Le Fey?
For me, it was Nimwe, if only because of what little control she had over her fate. Mab could at least try to contol people forgetting the Old Ways, for example. Nimwe is just a victim of circumstance. She comes across Merlin purely by chance, is taken hostage by Vortegan for no reason other than that he doesn't trust her father; she ends up physically deformed because she gets caught in the middle of Mab & Merlin's dispute, and spends years as a recluse in a convent, living in shame because of it. The one chance she has to make her situation better, she basically gets screwed over, being forced to spend years away from the man she loves. That Merlin basically pulls some weird parlor trick that makes the two of them young again is little consolation for the fact that Nimwe spent years alone and living in shame.
Her fate is even more tragic in the legend, as I understand. Doesn't she commit suicide?