Mikado Sanzen'in is Nagi's grandfather and the head of the Sanzen'in family. He lives in a mansion much bigger than Nagi's mansion with many servants(who are all same-looking). It is unknown where the Sanzen'in fortune comes from but Nagi calls her grandfather an oil tycoon. He has the ability to judge a person's character just by looking at them.
Even though Mikado is Nagi's only direct relative, they don't get along. Nagi dreads on going to the main house to visit him. As a result, Mikado is not seen after his first appearance. However, it is obvious that he worries for her when he tries to stop the assassination/kidnapping attempts and pushes them onto Hayate when he announces that he will pass the Sanzen'in inheritance to the person who makes Nagi cry and beg for forgiveness and later changed, with the help of Maria who he is fond of, to get rid of the butler first.