Beelzebub - Jitaku Keibiin

Tsunetaka Kareki. Codename: Noir Set.
He is a private home security officer with 20 years of military experience. Exceptionally trained in technology and intelligence gathering, he perceives and records the smallest of details of the home to which he is dispatched.

His new assignment is directly from his father, Takao Kareki. He must look after his father's new lover, a potential bride. Nothing but the best professional will do to keep her safe. But in her home, he sees all.

[Based on the DLsite English description.]