He was his cousin, for Percy is Poseidons son and Ares is Zeus son, and poseidon and Zeus are brothers, so this makes Percy and Ares cousins, And ares was also one of he villans in the lightning thief.
Percy is the son of Poseidon; his mother, Sally Jackson, married a man named Gabe Ugliano when Percy was young. Ugliano was abusive towards Percy and Sally, the latter of whom leaves him in The Lightning Thief .
