One Room ni Shitsuji.

A beautiful man suddenly turns up at Satori's run-down apartment, saying, "I am Saizo Kusunoki, your butler." In truth, Satori is descended from a super rich family. As a young child, though, he began to show signs of social withdrawal as a result of constant stress, so his mother, who was opposed to her family's privileged mindset, left home along with Satori and his father. The three of them lived a humble but happy life... And now, after his parents' sudden death, Satori lives as a poor student who spends all of his time working. Enter the butler. He is from the main family and informs Satori of the recent death of his grandfather along with the will which names Satori as the next family head... Satori receives daily training on how to purport himself from this handsome, older, aristocratic, slightly sadistic butler. Kusunoki calls it his "duty," and soon starts to train Satori's body, too!

(Source: Renta!)