Kinu Himuro

Kinu Himuro, nicknamed Okinu, is a ghost of an orphan teenage girl that was sacrificed 3 centuries ago to stop a volcano from erupting. A person that dies this way will usually become the guardian of the mountain, but Okinu wasn't qualified to become a celestial being nor a saint, and is stuck in the mountain until she finds a scapegoat. Being stuck to the land due to her long stay, wanting to go to heaven, Okinu makes a deal with Mikami, who may exorcise her in exchange of money, but as a ghost she doesnt have the money to pay for her own exorcism, so Okinu accepts to become the secretary and errand girl for Mikami, working for 30 yen per day. Because of her age (she's over 300 years old, even if her appearance the one is of a girl of 15 or 16 years old), and due to her ability to be perceived by the living, between the other friendly spirits she is called Senpai.