Thalia Grace is a Greek demigod, the daughter of Zeus and the mortal Beryl Grace, and the older sister of Roman demigod Jason Grace. She was transformed into a pine tree on Half-Blood Hill, which still stands as the official border for Camp Half-Blood to this day after being mortally wounded by attacking monsters. She is later resurrected by the Golden Fleece under the manipulations of Kronos in an attempt to control the Great Prophecy. She is currently the lieutenant of the Hunters of Artemis, leading them in the absence of the goddess after accepting Artemis' offer following a failed attempt by Kronos to use Thalia to destroy the gods.

Thalia has bright blue eyes and spiky black hair, and wears black eyeliner and punk style clothing. Jason describes her as having a Mediterranean complexion. Annabeth and Chiron remark that her personality and character traits (like her bravery and loyalty) are very similar to Percy's. She also shares some traits with her father, such as his pride, confidence, and vehement reactions to betrayal or contradiction. An incredibly skilled warrior, she is willing to attack even Luke, who was known as the best swordsman of the last 300 years. Thalia's weapons are the shield Aegis, disguised as a silver bracelet, and a spear disguised as a Mace can.