Also known as: Kai Chikanao (甲斐 親直)

Son of Kai Chikanobu and chief retainer of Aso Koretoyo, chief priest of Higo Province's Aso Shrine and head of the Aso Clan. His excellent military strategies and diplomatic maneuvering helped the Aso Clan retain its territories after one ally, the Ootomo Clan, suffered a catastrophic defeat to the Shimazu Clan in the Battle of Mimi-kawa and another, the Sagara Clan surrendered to Shimazu and was ordered to attack its former ally.

He died of illness in 1583 at the age of 75 (some said he was poisoned by his granddaughter, daughter of his heir Kai Chikahide). Souun's loyalty to the Aso Clan was such that he mercilessly purged any who tried to betray his family or disobey his family's policies, including killing his second and third sons and exiling his fourth for approaching Itou Yoshisuke of Hyuuga. In response Chikahide tried to assassinate his father, but his attempt was exposed. He only escaped death thanks to the pleas of Souun's vassals and because he was heir. Chikahide's wife's father had also been killed by Souun for disloyalty, and she was so terrified that she allegedly ordered her daughter to poison Souun.