Shia is a very polite and quiet girl who is living as a roomate to Misha. Though hard to see through her candy-coated exterior, Shia is actually a demon (and not a very good one at that, due to the fact she is kind-hearted and slightly weak). Shia is very domestic and excels at cooking and cleaning.
In the manga, Shia is more demon than anime: she even bites Kotarou in the neck, and causes him to almost die 2 times, the reason this being later explained that demons feel a greather atratcion to their oun family's blood. Demons can't survive in the human world for long, so they're forced to take the life from something else (people or animals) to continue living. To preserve her life, Shia was offered Misha's blood, both of them not knowing an angel's lifeforce is actually poisonous to demons, so Shia's life was being shortened in the long run. Shia didn't also say farewell to everyone, she only said them to Ayanokoji. In the manga, it is later revealed that she is Kotarou's great-grandmother. In the book 7, she uses her last powers and dies to the train station, right as she started considering her new life with her memory restored. Nya comes to pick up her body, and complement her on how good a demon she actually was at the end, causing a great scar in her friend's heart, and wasting both his and Taro's (her husband, Koutarou's great-grandfather) time waiting for her to come back.