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, causing him to be unable to help Lester to turn back into Apollo.
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 is giggling and drooling. She laughs every time Lester says Percy's name, and Lester says that if she stays as curious as she is, she could conquer the world, so he decides 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.