Ingo and Percy Jackson.
While vacationing in Las Vegas with her family and Roger, Sapphire stumbles upon the Lotus casino hotel and becomes trapped there.
Years later Percy, Anabeth and Grover go there during the Lighting thief and run into Sapphire, they become friends and when they leave they take Sapphire with them. Sapphire is shocked to discover she's been in there for years and ends up going with the group until she knows what to do. Sapphire has her own pearl that she found that she uses to escape the underworld with the group as well since she goes with them.
After the Lightning bolt is given back Sapphire decides to stay at camp half blood instead of going home because it would cause her family more pain to discover she's alive and unaged from the last time they saw her.
It's revealed that Sapphire is the granddaughter of Poseidon on her father's side.