Mikado Sanzenin is Nagi's grandfather and the head of the Sanzenin family, as well as one of the five directors in charge of Hakuō Academy. He lives in a mansion much bigger than Nagi's mansion with many servants. It is unknown where the Sanzenin 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. From what he saw in Hayate, he gives Hayate a pendant that is supposed to be his compass for life (but turns out to be a cursed pendant).
Even though Mikado is Nagi's only direct relative, they do not get along. Nagi dreads going to the main house to visit him. As a result, Mikado is not seen many times after his first appearance, although he makes a brief appearance in volume 12 when Nagi states that although she knows that no matter how much money she has, she cannot bring back her mother; there may still be some people in this world that do not understand this. However, it is obvious that he worries for her, which is shown when he tries to stop the assassination/kidnapping attempts and pushes them onto Hayate when he announces that he will pass the Sanzenin inheritance to the person who makes Nagi cry and beg for forgiveness and which later changed – with the suggestion from Hayate and a little help from Maria (who he is fond of) – to get rid of the butler first. This further changed in volume 21 of the manga, that the inheritance will be granted to the one who manages to steal or destroy the King's Stone Mikado gave to Hayate. In episode 51 of the first anime, he hatched a plan to fire Hayate, although it failed to materialize.
"Mikado" means emperor, showing his position in the Sanzenin family. There is a possibility that his name is a reference to the one-time villain Mikado Sanzenin in Ranma 1/2. In the Animax English dub, due to his dubious nature, he is given a British mobster-like voice.