Uphoria

Minori is a high school student in Kyoto that is very into occult/haunted/horror stuff. One day she takes a trip to the Aokigahara Forest to explore some haunted stories. She ends up walking through the forest for a while. Then, as she keeps going she starts to feel sick, she starts to get pains in her head and stomach which cause her to fall to the ground. When she awakes she no longer looks like herself, she has been possessed by a demon named Khoda. Khoda is a soul demon that was cast out from his home after being rejected by his father, the king of the soul demons. As a reaction to this rejection, he ends up Possessing a human. At first their souls fight for control, but they later make a deal to both gain control and they attempt to live cohesively.