Life for women in ancient Greece was hard - you had to fight for every inch of ground you got. Both Thetis and Briseis are strong, passionate women and in another time and place their lives would have been very different. Part of the tragedy of their characters is how much they have to offer - and how little of that they get to realize. Thetis spends the whole novel fighting the limitations placed on her, desperately trying to eke out the best she can from a bad situation. This makes her fierce and terrifying.