Otto no Jijou Tsuma no Himitsu

Michel is the third son of the Norman family, who have been in the spy business for generations. The Runo clan act as the Normans' servants in public, but secretly also work as secret agents themselves. Michel decided to marry his maid Coco Runo purely as an act of convenience, but he slowly becomes attracted to her even though he doesn't realize his housekeeper's true identity.