Shiemi is Rin's classmate and friend. Before she met Rin, Shiemi tended her grandmother's garden while her mother ran an Exorcist-only shop inside True Cross Academy. During her encounter with Rin and Yukio, they discovered that Shiemi was having her life force leached out by a Demon. After Yukio exorcised the demon, Shiemi finally decided to become an Exorcist and started to attend class. It was later revealed that Shiemi had the talent to become a Tamer, as she was able to summon a Greenman Spirit during class. It is apparently a rare talent amongst the Exorcists, where there are only a few who can control demons and fight. She seems to have a liking for both Yukio and Rin but she started avoiding both after she learned that they are sons of Satan, because -- as they kept their origin a secret even from her -- she blames herself for not being trustworthy enough.
Informations Source: Wikipedia