Ep. 13: Proof
Shura, a high-level Exorcist and former apprentice of Shiro, posed as one of Rin's classmates under the name "Yamada" because she was tasked to investigate his connection with Satan. It is shown that Shiro requested Shura to look after Rin in his place if something should happen to him. After stealing Kurikara, Shura draws her own sword out of her chest and attacks him. With orders to kill Rin if his connection with Satan was proven true, she is divided between fulfilling her obligations and following Shiro's request. Her doubts are cleared when she confronts Rin and finds that Shiro was actually raising him as a son instead of a weapon. Rin, recalling during his childhood of when he hit a boy for calling him a demon, remembers how Shiro comforted him and took in all the pain he caused, proving himself as a good father. Shura, holding on to Kurikara, ultimately decides to start teaching at the academy in order to secretly train Rin. (Source: Wikipedia)