Medusa is a monster, initially disguised as a friendly woman named Aunty Em, who lures Percy, Annabeth, and Grover into her establishment at the start of their quest. Medusa tries to attack the children, in part because she has a vendetta against Annabeth’s mom, Athena. According to Greek mythology, Athena, the goddess of wisdom, punished Medusa by transforming her into a hideous monster with snakes for hair and the ability to turn her victims to stone whenever they look her in the eye. The trio’s battle with Medusa is significant because, unlike the incident on the bus in which they are almost killed by the Furies, it is the first time that the three of them are able to truly work together to defeat a common enemy. The battle with Medusa is essential to Percy, Annabeth, and Grover’s development; it shows that they are learning to become a team. 

The three children are left with Medusa’s severed head at the end of their fight. For the rest of the book, the head becomes a symbol of agency and free will. Percy “impertinent[ly]” mails Medusa’s head to Olympus to show the gods that he is not afraid of their power and that they do not control him. Medusa’s head returns at the end of the novel when Sally uses the head’s powers to turn Gabe and his friends into a statue, which she sells for a large profit, thereby taking control of her own life to rid herself of the man who abused her for so many years.