Milli was the black sheep of not one, but two prominent Watcher families, who had signed up to learn with the Watchers on her own terms following her estrangement from her family over their attempts to control her.
She was reckless, quick to anger, unused to being around people her own age as she attended the University of Cambridge as part of her studies, and also in love for the first time...
She died saving the rest of her group from a telekinetic storm of weapons controlled by the villainous brother of another group member, who had given her a bracelet enchanted to stop time, enabling her to catch or deflect almost all of them. The last one, a poisoned dagger aimed at her first love, caught her in the back as time reasserted itself and she pushed him out of the way.
She died in her mentor's and her best friend's arms, as her beloved fought the villain.
She got better thanks to the events around her death causing a split in reality, but that's another story.