In Duma Key, Edgar sees the ghost ship as being named "Perse". He assumes it's short for "Persephone", and refers to the Big Bad as "Perse". However, if the name for the entity is taken from the ship, instead of being given to it, the ship could've just as easily been called the Perseus. I believe Perse is described having snakes for hair and a paralyzing gaze, much like Medusa. I doubt even King himself realized he was opening up the possibility of an Alternate Character Interpretation, though. — Jonn
Perse's the cause of Elizabeth's Alzheimer's. When Edgar's show approaches, Perse shuts her down so that there will be no interruptions getting the paintings off the island. She then releases Elizabeth to keep Edgar from analyzing his "Girl and Ship" paintings and realizing what they mean, and then kills Elizabeth to further distract Edgar and give her time to work on Tom, Kamen, and Mary.