Who is Ella in the novel, The Tyrant’s Tomb?

Ella the harpy is one of the protectors of the Sibylline books within New Rome and is always seen with Tyson. It is revealed early on that Ella’s photographic memory allowed her to memorize all of the Sibylline books. Unfortunately, she is only able to write them in order as tattoos on Tyason’s skin. Furthermore, she struggles to focus on conversations, as her mind frequently makes links between what is being said and a random fact she remembers.