Miyoshi Masayasu served the Miyoshi clan during Japan's turbulent Sengoku period. He was a key member of the Miyoshi Trio, the Sanninshu, alongside Miyoshi Nagayasu and Iwanari Tomomichi. After clan leader Miyoshi Nagayoshi died, Masayasu and the Trio became guardians for the young heir Yoshitsugu, managing clan affairs in collaboration with Matsunaga Hisahide. Masayasu participated with the Trio and Hisahide in overthrowing Ashikaga Yoshiteru in 1566. Conflict later erupted between Masayasu and Hisahide, who sought to eliminate him by allying with the Tsutsui and Hatakayama clans. These attempts failed. Masayasu countered by forging alliances with the Ikeda and Ataka clans against Hisahide. He fought prominently at the battles of Kumeda and Kyoukouji, earning recognition in clan records for his contributions. During the Battle of Rokujō in January 1569, Masayasu and the Miyoshi Trio attacked Ashikaga Yoshiaki, then under Oda Nobunaga's protection. They assaulted Honkoku-ji Temple in Kyoto, aiming to exploit Nobunaga's absence. Initial momentum faltered when reinforcements led by Hosokawa Fujitaka and Miyoshi Yoshitsugu arrived, repelling the Miyoshi forces and forcing their retreat and defeat. Following the Battle of Shōryuji, Masayasu surrendered Kozu-jo castle under orders. He subsequently vanishes from historical records, his ultimate fate undocumented. Contemporary accounts by Luís Fróis criticize Masayasu for his harsh treatment of Christian merchants in Sakai, attributed to his opposition to their faith. In game adaptations, Masayasu appears defending Azai positions during Oichi's story mode in Samurai Warriors 2 and later fights Tokugawa forces at Ōsaka Castle. Samurai Warriors 4 features him at Rokujō and Noda-Fukushima; he attempts to retreat when battles turn unfavorable, requiring specific mission objectives to intercept him. At Rokujō, he joins the sudden attack on Ashikaga Yoshiaki instigated by Hisahide Matsunaga. Kessen III portrays him as excessively supportive of Nagayasu's viewpoints and dismissive of Nobunaga, frequently maneuvering to flank opponents targeting Nagayasu during battles like Shōryuji and Noda-Fukushima.