A daimyo of Higo Province and chief priest of Aso Shrine, Aso Koremasa was the son of Aso Koretoyo and became head of the clan upon his father's death in 1559. Koremasa relied on Kai Souun in the midst of aggression from the Shimazu and Ryouzouji clans. He also formed an alliance with the Ootomo Clan. His younger brother Aso Koretane became clan head after his death in 1583.