Kestrel

Kestrel "Kes" West was Peregrine's younger sister and Ambrose's friend who died back in the 1920s, and she was the one who found the spellbook that Peri now carries with him. She was a kindhearted young woman who was always looking toward the future, and she had a love for the morbid and grotesque. 

Born sickly, she pursued a cure through life magic, developing magical potions that could heal all kinds of maladies. Unable to find one and with her condition rapidly deteriorating, Kes used Peri's research into death magic to create a resurrection potion that turned her into the first lich. But because this potion was so experimental, the magic sustaining her became unstable. She sporadically lost control of herself and her physical form, and despite the group's best efforts to fix the spell, she finally mutated, body and mind, into a necromantic beast.

In the ensuing struggle, the creature that was once Kes killed Ambrose and Peri had to kill her before anyone else got hurt. Peri took Ambrose's body and fled, later reviving him as a lich as well. He had figured out how to stabilize the spell, but it was just a little too late to help Kes.

Both Ambrose and Peregrine miss Kestrel a lot, but they don't talk about her.