Older step-brother of Sagiri. Masamune is a light novel author and a high school student at the same time. He wants his sister who is shut in her room all day to come out. Masamune is very reliable because of the responsibility he had to overtake, due to the accident of their parents. For his light novels, Masamune hired an anonymous illustrator named "Eromanga Sensei". When it came out that Masamune and Sagiri were already working together, the step-sibling relationship reaches new levels of excitement. Masamune's rival is Elf Yamada, even though he is a fan of her light novels, because of the emotional story, he had to go through. But the relationship between Masamune and Elf develops throughout the Anime.