The information in this article is from the fan game King's Quest II: Romancing the Stones and is therefore not canon.
Lady Lillian was the mother of Possum. Killed by the monks and their leader. Now exists as a ghost guarding the castle. She died in 1668 according to the KQ2 (AGDI) timeline.
Behind the scenes[]
She is based on the ghosts in front of Dracula's castle in KQ2:Romaching the Throne.