Hisau Maiya is a young woman who acts as Emiya Kiritsugu's assistant during the fourth Holy Grail War. She is cold and professional, which makes it easier for Kiritsugu to act as he needs to.
Little is known about her history, save that Kiritsugu had rescued her from a war-torn land many years ago. When Maiya was a child, she was forced into the army and trained to be a soldier. She later reveals to Irisviel von Einzbern, Kiritsugu's wife, that she had been raped by other soldiers and later lost her child. After rescuing her, Kiritsugu became Maiya's mentor and taught her his way of life. Her real name is unknown; her current name was given to her by Kiritsugu when he created identity documents for her.
When Kiritsugu was hired by the Einzberns to obtain the Holy Grail eight years before the Fourth Heaven's Feel, he entrusted Maiya with all his former equipment - in particular, the gun which allows Kiritsugu to employ his Mystic Code.