Lucretia Constance

Lucretia (never Lucy) is an honest-to-God improbable nexus child resulting from an unlikely series of events involving time-travelling heartthrob Jack Harkness, a school, the souls of unbaptized babes, robots, Severus Snape, and the demon halfbreed Balthazar. Really, that summary makes as much sense as the actual story and uses fewer words. Lucretia was given to Balthazar, who was bound by his word and Snape's magic to raise her "well."

Let's watch, shall we?