Percy mentioned his unborn half-sister to Apollo. He said that he wanted to meet his sister when she is born and be there for her when she is in need. This was also the reason he could not help Lester to turn back into Apollo(or be in Trials of Apollo very much)
Estelle appears in the Jackson's apartment when Apollo, Meg McCaffrey, and Luguselwa arrive looking for Percy's help. Her father is bouncing her on his lap. She was giggling and drooling. She laughed every time Lester said Percy's name, and Lester said that if she stayed as curious as she was, she could conquer the world, so he decided not to tell Zeus about her. She is put in bed as the others prepare for dinner. In the morning she is awake when Apollo, Meg, and Lu leave.
Estelle is a baby girl with sea-green eyes and salt and pepper hair.
- Estelle is a feminine given name meaning "star" that originates from the southern region of France and surrounding countries known as Occitania.
- Estelle is named after her maternal grandmother.
- As Sally Jackson has Clear Sight, the ability to see through the Mist, Estelle may inherit this ability as well.
- Given that Sally was seven months pregnant in The Hidden Oracle, which takes place in January, Estelle was probably born in or around March.