Natsume's grandmother, from whom he inherited the ability to see spirits. When Reiko was young, she was considered a freak by everyone else because of her ability. Since she believed no human could ever understand her, she turned to the spirits for companionship. Reiko's power was extraordinarily powerful, and so she bullied spirits in to obeying her by playing games with them. If they lost, she would make them give her their names. Reiko gathered the names in to the Book of Friends, and commanded most spirits, except for Madara, who beat her in a game. Reiko died when she was young, so no one remembered her.
an extended description!