Proof.Shura had been a former apprentice of Shiro, and posed as one of Rin's classmates because she was tasked to investigate his connection with Satan. Shiro had requested Shura to look after Rin should something happen to him. With orders to kill him if his connection is proven true, she is divided between fulfilling her duty and keeping Shiro's request. Her doubts are cleared when she discovers that Rin loved Shiro as his father, and Shiro was actually raising him as a son, not a weapon against Satan.