Thalia Grace is a fictional supporting character in Rick Riordan's "Percy Jackson and the Olympians" series, as well as a minor character in the Heroes of Olympus, as Jason's sister. She is the demigod daughter of Zeus and Beryl Grace, a famous mortal actress. Thalia has angel-blue eyes ( thought done say she has stormy green), black long hair, and a spray of freckles and a brilliant singing voice.