Makai Ouji - Devils and Realist

Synopsis

William Twining is one of Stratford School's top students, but despite attending a religious school, he does not believe in God or the supernatural because he is an emphatic realist and a proponent of modern science. Although the Twinings are one of Britain's most prestigious noble families, William's missing uncle accumulated a lot of debt. One day, desperate to find anything valuable in his manor that he can sell to pay this debt, William stumbles upon a secret room in which a stranger inexplicably appears. This intruder introduces himself as Dantalion, Grand Duke of Hell. Dantalion claims that William is the "elector" who can choose the representative Emperor of Hell, and furthermore, this authority is inherited only by descendants of the ancient wise king Solomon. William's mindset does not allow for the existence of Heaven or Hell, so naturally he denounces Dantalion as a fraud and declines to support his candidacy for Emperor. However, the demon refuses to take no for an answer, even if he must enroll at Stratford as "Dantalion Huber" in order to stay near the elector and convince him. How can William fend off this unwelcome hanger-on, and the others that are sure to follow?

Synopsis & details from MangaUpdates