Sometime before the Peloponnesian War, Harpalos found a group of the Followers of Ares hiding in tombs, and they welcomed him with open arms. His way of preaching appealed to the Followers, and he became their leader. Under him, the Followers gained the blessing of the Cult of Kosmos, and became an army for the Cult.
Harpalos stayed at the Cave of Ares on the island of Keos, and sent out notes to the cult to gather there. One of these notes ended up in the hands of the Spartan misthios Kassandra, who had sworn to destroy the Cult of Kosmos. Directed by the note, Kassandra located the cave on Keos, and slew Harpalos.
Harpalos, a character in Assassin's Creed: Odyssey, shares his name with multiple characters in Greek antiquity, among them a fifth century astronomer. In Greek mythology, Harpalus was the name of the son of Amyclas, king of Lakonia and father of Deritus, ancestor of Patreus, founder of Patras.
In Assassin's Creed: Odyssey, Another cultist carries a letter that serves as a clue to Harpalos' identity and adds his masked icon to the Cultist branch. However, it is far more likely that the player will have already collected the "necessary" clue to wholly unveil him by killing (and looting) any Follower of Ares in the world, before completing Ashes to Ashes. If that clue is collected first, the former's letter cannot be read in Harpalos' submenu. If the player ignores all Followers of Ares until after the former cultist is dead, both clues can be read in Harpalos' submenu. The letter will still be available to read under the submenu for the branch's Sage, provided she herself has not been killed prior to being unveiled.