Jason Grace is a son of Jupiter and the mortal Beryl Grace, and younger brother of Thalia Grace.

Jason is described as having blond hair, blue eyes, and a scar above his upper lip (according to Thalia, this is the result of an attempt to eat a stapler when he was two). He is of above-average height, with an athletic build and muscular arms. He has a tattoo burned under his forearm, with twelve straight lines like a bar code, an eagle, and the letters SPQR over the lines. The tattoo symbolizes his twelve years of service in the Twelfth Legion Fulminata at Camp Jupiter; the eagle is a symbol of Jupiter. By the age of fifteen, he had earned the rank of praetor and led the legion with his longtime quest partner, Reyna. Jason also coordinated the Roman camp's attack on the Titan force. He led an assault against Mount Othrys, a Titan stronghold near San Francisco, and defeated the Titan Krios in combat, much as Percy Jackson defeated Kronos.

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