Lillian Simms was Katherine Mayfair's aunt.
Lillian has to go and live in a nursing home, due to her failing health, and rents her house to Mike Delfino.
When Carlos Solis divorces Gabrielle, he needs a place to stay and Mike offers him to come and live for with him for a while. However, Edie Britt goes and sees Lillian at the nursing home and tells her he is a drug addict. Lillian then refuses to let Carlos live with Mike and sends her son to evict Carlos.
Lillian is brought back to Wisteria Lane, to die. Katherine lets her stay at her house to care for her during her last days. Lillian starts to feel very guilty (and fears of going to Hell) unless she and Katherine tell the police what they did. Katherine refuses. Lillian then tries to tell Dylan Mayfair her secret, but Katherine bullies her and she doesn't. Later, Lillian is struggling to breath and Katherine makes Dylan practice her cello, so they don't have to hear her die. Lillian writes a note to Dylan, but she dies, and the note slides under the bed. Her body is later found and taken away in the body bag.
Lillian's ghost watches Susan leave Wisteria Lane. ("Finishing the Hat")
When Susan Delfino decides to leave Wisteria Lane with her children, MJ Delfino and Julie Mayer, and granddaughter, Lillian's ghost appeared alongside Rex Van de Kamp, Beth Young and Chuck Vance, in front of 4354 Wisteria Lane, and they watched the family as they drove away from the lane and began their new lives together. ("Finishing the Hat")