Pallas is the daughter of Triton and a nymph. She treats Athena like she's part of her family when she was stuck on Earth, not ready to go back to Mount Olympus yet. In the mythology, she is an immortal as well. She was sparring with Athena when Zeus got scared that Athena would get hurt and showed Pallas Medusa's face. Pallas was so stunned at the ugliness of Medusa's face that she was unable to break her gaze and block Athena's stab and died. Athena, in grief of losing her best friend, built a temple and statue so the Greeks could worship Pallas too.
Pallas is a warm and welcoming person. She welcomed Athena in her home and still treats her like a sister.
Pallas is a helpful and kind mortal in the book. Although she only got one appearance throughout the series, Pallas and Athena still vow to remember each other.
Pallas' official Greek name is Παλλας.