Lifespan: Meiō 2 (1493) to 11/7 of Eiroku 2 (1559)
Rank: bushō; sengoku daimyō
Title: Junior Second Rank, Chief Priest of the Aso Shrine
Father: Aso Korenori
Siblings: Korenaga (later known as Kikuchi Taketsune), Koretoyo
Children: Daughter (formal wife of Nyūta Chikazane), Koremasa, Koretane
Aso Koretoyo served as a sengoku daimyō and the eighteenth head of the Aso clan of Higo Province. Koretoyo further served as the chief priest of the Aso Shrine in Higo Province. The shrine had the status of an ichi-no-miya meaning the highest level of shrine in the province. This made the shrine eligible for support from the Imperial Court. Koretoyo led the clan during its peak years of prosperity with the support of Kai Chikanobu and his son, Kai Chikanao (later known as Kai Sōun). Koretoyo was one of the Aso go-ka-shoshū, a group of five leaders in the clan.
In 1493, Koretoyo was born as the son of Aso Korenori, the sixteenth lineal head of the Aso clan. In 1505, he inherited the Kikuchi clan, the shugo, or military governor of Higo. Koretoyo’s older brother, Aso Korenaga, had earlier taken over the clan and changed his name to Kikuchi Taketsune, assigning to Koretoyo the role of chief priest of Aso Shrine. Korenaga, however, Korenaga had ambitions to reclaim the Aso clan. In 1513, he conspired with the Shimazu clan to attack Koretoyo, causing Koretoyo to flee to Hyūga Province. Korenaga appointed his eldest son, Koresaki, to serve as chief priest of the Aso Shrine in place of Koretoyo.
In 1517, Koretoyo received support from Kai Chikanao, a kokujin, or provincial landowner, from Kuraoka in Takachiho, to recapture the mountainous Yabe area of the Kamimashiki District that was the home base of the Aso clan. The strife between Koretoyo and Korenaga and his son, Koresaki, continued unabated. In 1523, Koretoyo lost Katashida Castle to Korenaga and Koresaki, along with the villages of Kōsa, Tomochi, and Nakayama. In 1543, Koretoyo reclaimed Katashida and routed Koresaki. Koresaki and Korenaga fled with the assistance of the Sagara clan. Divisions in the Aso clan that had persisted for three decades headed toward a final chapter.
In 1540, Koretoyo received scriptures written by Emperor Gonara and kept them in the building located on the highest ground of the Aso Shrine. In 1549, Koretoyo made an offering of 10,000 hiki (either 100,000 or 250,000 mon) for repairs to the Imperial residence, and, in return, Emperor Gonara awarded Koretoyo that title of Junior Second Rank. His daughter became the formal wife of a senior retainer of the Ōtomo named Nyūta Chikazane. After an internal conflict within the Ōtomo clan known as the Nikai kuzure no hen, or Collapse on the Second Floor, Koretoyo offered protection to Chikazane, but Chikazane was despised as one of the ringleaders of the incident, and later killed.
Koretoyo entered into alliances with the Ōtomo and Sagara clans to bring stability to his dominion, but died in 1559. Following a major defeat by the Ōtomo at the Battle of Mimikawa in 1578, the kokujinshū, or clans of local influence in Higo, formed relations with the Shimazu and up-and-coming Ryūzōji clans, posing a threat to the Aso territory. Aso Koremasa, with the exceptional military strategy of Kai Sōun, managed to protect their lands, but, by 1581, the Sagara had surrendered to the Shimazu, who were advancing from the south. The successive deaths of Koremasa, his successor (Aso Koretane), and Sōun between 1583 and 1585 left the Aso clan in a weakened and vulnerable condition.
In addition to the external threats to their existence, conflict within the Aso clan lasted until 1590, when Aso Korekata pledged loyalty to Aso Koremitsu to bring a formal end to their differences.