Yamanko!

The following content is intended for mature audiences and may contain sexual themes, gore, violence and/or strong language. Discretion is advised.

From SCX-Scans:

Though it was a bit strange, three people ended up meeting. One, who looks younger than her age and able to see the supernatural. Another who is possessed by a "Yamanko" which refers to "Yama no ko", or a mountain child. She was possessed due to her bloodline and due to the possession, she has superhuman abilities which she can call upon. Finally, an awkward person who enjoys writing poems. Coincidentally, the three are also classmates.