Endō Naotsune served as a bushō during the Sengoku and Azuchi-Momoyama periods. As a retainer of the Azai clan of northern Ōmi Province, Naotsune was known as a brave and intelligent commander.
The ancestors of the Endō clan were said to have originated from Kamakura warriors, and during the Kamakura period, left the capital after receiving territory in Õmi Province. Naotsune originated from Kashiwabara-no-shō in the Sakata District of Ōmi. The clan governed the area around Mount Sukawa for generations from Sukawa Castle. Similar to the Akao clan, the Endō were multi-generational retainers since the time that the Azai were hikan, or servants, of the Kyōgoku clan.
The Endō were allowed to have a residence in the Kiyomizu Valley below Odani Castle. Naotsune himself delegated the castle to his children and quartered in the residence. Naotsune helped raise and educate Azai Nagamasa, serving as a counselor to Nagamasa from an early age. As a consequence, Nagamasa closely trusted Naotsune, so when Nagamasa decided to attack the Rokkaku clan, he directly consulted with Naotsune and Azai Masazumi.
Naotsune quickly discerned the phenomenal abilities of Oda Nobunaga. In 1568, when the Azai and Oda families maintained an alliance, Naotsune was ordered to receive Nobunaga when Nobunaga visited Sawayama Castle in Õmi. On this occasion, Naotsune recommended to Nagamasa to devise a plan to assassinate Nobunaga. However, Nagamasa (and his father, Azai Hisamasa) rejected the idea on the basis that it violated principles of good faith. Naotsune would not disregard the orders of his lord, so the plan ended without an attempt.
As the relationship between the Asakura and the Oda clans deteriorated, the Azai were compelled to choose whether to honor their long-standing ties with the Asakura or the marital relationship with the Oda, whereupon Naotsune emphatically asserted that the Azai should side with the Oda. In this regard, Naotsune lost patience with Asakura Yoshikage, the head of the Asakura family, owing to Yoshikage’s indecisiveness. Moreover, Naotsune highly evaluated Nobunaga’s diplomatic prowess based on his interactions with him while representing the Azai as a senior retainer. In the end, Nagamasa adopted the recommendation of his father, Hisamasa, and others to side with the Asakura clan. As a result, Naotsune’s advice was disregarded.
In 1570, as the overwhelming defeat of the Azai army at the Battle of Anegawa became clear, Naotsune carried the head of an allied commander, Mitamura Saemon, into the enemy base while posing as a commander of the Oda, intent on assassinating Nobunaga. However, he was discovered by Takenaka Hisasaku (the younger brother of Takenaka Shigeharu) and his guardian, Fuwa Yaashi just twenty or so meters from Nobunaga’s main camp. He was then apprehended and executed. His head was awarded to Takenaka Hisasaku. The Takenaka brothers recognized Naotsune’s appearance after having earlier served as guest retainers of the Azai family following their separation from the Saitō.