Takashi Natsume's grandmother, from whom he inherited the ability to see yokai. Reiko had high spiritual energy, which gave her the ability to see and seal yokai.

Reiko was always shunned and even got rocks thrown at her for being able to see yokai. That is why she created the Book of Friends, Reiko was lonely and wanted friends. Since she believed no human could ever understand her, she turned to the spirits for companionship. Reiko was extraordinarily powerful, and so she bullied spirits into obeying her by playing games with them. If they lost, she would make them give her their names. Reiko gathered the names into the Book of Friends, this gave her the ability to summon whichever yokai she had the name of. A necessary part of the sealing was her saliva and breath. Madara mentions that she had sloppy table manners and was very forgetful.

Reiko died when she was young, so no one remembered her.