Willow was Marc's little sister. She died at Kithorn during the Kithorn massacre, when the Merikit slaughtered all of the keep but for Marc, who was out hunting. She hid in the keep's still room, underneath the still's boiler, and died there, of shock or starvation or smoke, and her soul remained, because her remains were not consecrated to the pyre. 80 years later, Torisen, accompanied by Burr, hears the pattering of her ghost's footsteps and searches for her remains, finding them and taking them with him.
In Peshtar, Mother Ragga hears the sound of Willow's feet in the Kithorn dirt, and tells Marc that it was only two days ago that those feet ran. Marc is stunned; he thought that she must have escaped or been carried off, because he searched for her remains "more times than [he] can remember".
Torisen finds himself reluctant to give her to the pyre, and her ghost goes with her bones, visible mostly in the shadows cast by fires and lamps. He takes her remains with him, keeping her by his side throughout the events that unfold.
When the Northern Host is encamped in the White Hills, Willow comes to Kindrie and makes him go along when Torisen goes out alone at night, ending up at the Keep in the Haunted Lands by some form of crossover, some correspondence. Tori and Kindrie follow Willow's footsteps to return to the camp.
Later, Tori falls into an unconscious state when Jame is drugged with wyrm's venom by Tirandys, and cannot be woken. Kindrie uses Willow to help find Torisen in the soulscape, and ends up drawing the poison from Jame's veins through the soulscape, and then having the vile stuff sucked from his own veins by the snakes of the Serpent-skin Cloak.
After the Battle of the Cataracts, Ashe takes Willow's bones to return to Marc. Willow shows up again in To Ride a Rathorn, when Marc brings her remains to Jame as she recuperates after binding Death's Head and losing huge amounts of blood. They are introduced, and Jorin plays with Willow in the lamp-light. Later, after Jame falls asleep, she encounters Willow in the dreamscape; Jame asks Willow to help her travel from the dreamscape to the soulscape, and it seems that Willow does help her.
During that time, Marc admits that he should give her bones to the pyre, but that it's hard, and he wants too keep her. Jame suggests he ask Ashe about it, and Marc agrees, though it's not known if he ever does. Again in Bound in Blood, Torisen asks Marc about burning her, and he says he will, but not quite yet. At the end of Bound in Blood, Marc melts Willow's bones into the piece of glass for Kithorn in the stained-glass map.