Willow Nigmos (née Muenda) is the deceased sister of Olive Specter. Willow was married to Creon Nigmos and had a daughter with him, Ophelia.
She is deceased before the game begins, and it is said that she drowned. After her death, Willow's daughter, Ophelia, goes to live with her aunt, Olive Specter (Willow's sister). At the start of The Sims 2, Olive is an elder, and Ophelia is a teenager, and they live together in their eerie home in Strangetown (which happens to be haunted by more than a few ghosts). As a ghost, she looks randomized, and her thumbnail will change. The same goes for her husband, Peponi Muenda, Lerato Muenda, Melissa Sims, Vicki Sims, Lyla Grunt, and Luc Smith.
It is best not to resurrect her, as you could ruin your game very badly she could be seriously weird looking or she could cause corruption throughout strange town.
Trivia
She is very glitchy when resurrected, her face is badly corrupted, and appears deformed and changes constantly. She will be bald, with green eyes instead of brown. It's impossible to change her back to normal.