Mukami Ruki

Ruki Mukami is the eldest son of the Mukami household. He is known as the 'Brains' of the Mukami family.
He always has an intimidating smirk on his face matched with a calm tone, but a sense of brutality can be felt in his spoken words. Without dirtying his own hands, he interacts with others as a cruel-hearted, military do-S (extreme sadist). Ayato notes that he is an "intellectual" person.