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Masahide first served Oda Nobuhide. He was talented not only as a samurai but also in various arts like sado and waka, and this helped him to act as a skilled diplomat, dealing with Ashikaga shogunate and deputies of the emperor. In 1533, a well-known regent Yamashina Tokitsugu made a visit to Owari Province, the dominion of Oda clan. Finding the superbness of the reception Masahide prepared, Tokitsugu highly praised Masahide's knowledge. Another evidence of his importance as a diplomat can be found in the fact that he paid a visit to Kyoto to offer the cost needed to repair the Emperor's residence in the name of Nobuhide.
When Nobuhide's son Nobunaga was born (1534), Masahide became the second highest ranking Karō as well as the tutor of the newborn heir. In 1547 Nobunaga finished his coming-of-age ceremony, and on the occasion of his first battle, Masahide served beside him. The next year, he exerted himself to establish peace between Nobuhide and his agelong rival Saitō Dōsan of Mino Province, and to arrange the marriage between Nobunaga and Dōsan's daughter Nōhime. This move made it possible for the Oda clan to concentrate on the fight against Imagawa clan.
In these ways, Masahide served the Oda family faithfully, but he was also deeply troubled by Nobunaga's eccentricity. After Nobuhide's death, discord in the clan increased and so did Masahide's concern about the future of his master. In despair, Masahide killed himself in 1553.
The suicide of Masahide is commonly known as the story of wise retainer remonstrating his young master with his own death. However this may not have been the true story. Some believe that Masahide felt responsible for Nobunaga's behavior and took responsibility with his own life. Others believe that after Nobunaga requested Masahide's son's horse and Masahide declined, Masahide faced a difficult situation. Still others think there were serious conflicts with other retainers. Nobunaga did not become less erratic after Masahide's death, but he did grieve the loss and built Seishu-ji (政秀寺?), a Buddhist temple dedicated to Masahide.