He's friend with the Mutou siblings (especially Tohru) and hates Natsuno. Masao is an insensitive, cowardly individual who has a childish demand for everyone's attention; his sister-in-law notes that he hates his nephew because before his birth Masao was always the one spoiled by the family.

Spoiler

He's bitten by the Shiki-turned librarian of Sotoba and also becomes a Shiki. On the night of the start of the revolution of the villagers against the Shiki, he begs to Megumi to run away with him, and Megumi immediately refuses, noting that he only wants her to come along in order not to be the only one who disobeys the Kirishiki's orders. In the anime, Masao, during the time of the human mob, attempts to run to his sister for cover, yet she beats him over the head with an iron pole and then proceeds to stake his chest. In the manga, Masao tries to seek refuge as the shiki extermination continues. After he first thinks about his father and brother, he concedes that only his sister will help him. When he arrives home, he pleads to her niece to allow him inside. Thinking she is alone, he plots to feed on her and his family, thinking he can assume control over them. As he enters the house, Chizuko incapacitates Masao after she trows a cleaver at his head. As he lays hurt, Chizuko reprimands him as she proceeds to stake him