Satoshi Sakai is Shizuka's brother. He was dying due to unknown causes, but he pushed himself to complete a project to secure his sister's freedom, which led to his premature death during the prologue. Satoshi's appearance in the story is very brief, but in that time he makes it clear that he feels responsible for getting Shizuka involved in some bad business and that he is willing to do whatever he can to protect her, even if it means his own death. He is likely very intelligent, given how essential he is to the project he was completing.