Thalia Grace
There are two parts in which she is either mentioned or appeared. First, in Percy's dream which was a flashback of him in The Titan's Curse, Thalia, Percy and Zoë Nightshade battling on Mt. Tam going against Atlas, Luke, and trying to save Annabeth and Artemis. Percy later sees Zoë dying and rise to the stars. Second, though Thalia was never mentioned, when Hylla, the Queen of the Amazons, talks to Hazel Levesque she mentions a group of archers that travel around who follow the goddess Artemis, which refers to the Hunters.