The Girl From the Other Side: Siúil, a Rún

In a world split between the Inside and the Outside, those living in both realms are told never to cross over to the other side, lest they be cursed. A young girl named Shiva lives on the other side, in a vacant village with a demonic guardian known only as “Teacher.” Although the two are forbidden to touch, they seem to share a bond that transcends their disparate appearances. But when Shiva leaves Teacher’s care to seek out her grandmother, the secret behind her mysterious living arrangement comes to light.

(Source: Seven Seas Entertainment)

Note: Bundled with the limited edition of the manga's 8th volume.