Isao Mutsu (陸奥勲雄, Mutsu Isao) is one of the three suspects of the murder case of the victim Kaizō Fūra. He is a good friend of his neighbor Nanami which he came up with the detailed information about the death of Fūra when Takagi was surprised about it. Isao works at a company of building maintenance which he explains what the company is for.
He is seen to be a calm man that can be easily scarred about things he doesn't like or wanted to happen. It seems that he is a person with a good personality, which he admit that he had a grudge against Fūra, but it wasn't that far to kill him, which a normal person should kill him directly.