Reiko Natsume, maternal grandmother to Takashi Natsume, possessed exceptionally strong spiritual powers allowing her to see, hear, and combat yokai. These abilities caused her lifelong ostracization by humans. Fearful children threw rocks at her, leaving her with torn clothes, unkempt hair, and bruises, deepening her isolation and complex relationships with both humans and supernatural beings. Her background remains largely undefined, though she was likely orphaned or passed between relatives during childhood, mirroring Takashi's experience. She never married, retaining the name "Natsume." In her late teens or early twenties, a relationship with an unidentified man resulted in the birth of her daughter (Takashi's mother) out of wedlock. The father's absence, whether by departure or death, fueled rumors. Her daughter, described as a "normal, quiet person," likely lacked spiritual abilities. Reiko raised her alone but died young under a tree when her daughter was still young, leaving few human memories. The exact circumstances of her death are unknown. Physically, Reiko strikingly resembled Takashi, sharing his dusty-blonde hair (silver in the manga) and amber eyes (pale green in the manga), with distinctive cat-like pupils. She typically wore a sailor-style school uniform and maintained a perpetual slight smile. Flashbacks consistently depict her as a teenager, reflecting the period of her most significant yokai interactions. Reiko's loneliness and longing for companionship drove her personality. Perceived as a bully toward humans and yokai, this behavior served as a coping mechanism for her isolation. She frequently challenged yokai to games or duels; upon winning, she forced them to surrender their names, which she recorded in her creation, the Book of Friends. This book granted her command over the named yokai. Despite her confrontational approach, she occasionally showed unexpected kindness—returning a stolen doll to a child or releasing yokai like Kibune from contracts—though humans often misinterpreted or rejected these gestures. Her forgetfulness left many yokai awaiting her return decades later due to unfulfilled promises. She also exhibited traits like poor table manners, noted to have caused pages in the Book of Friends to stick together. Her history centers on the Book of Friends, created to manage the names of subdued yokai. This artifact became highly sought after in the supernatural world, drawing conflict that extended to Takashi after her death. Reiko spent most of her time engaging with yokai, though she formed occasional human bonds, such as with the sickly Sōko Morinaga, who inspired her to duel yokai. Her spiritual prowess was formidable; she banished the yokai Karime from the Fujiwara household and negotiated with entities like Hinoe for safe passage. Her relationship with the powerful yokai Madara was notable; though he dismissed her as a "bully," he could imitate her appearance and mannerisms and acknowledged her as one of the few humans he respected. Their interactions suggested a close connection prior to her untimely death. Reiko's legacy is defined by the Book of Friends and its impact on Takashi, who inherited both her spiritual powers and the task of resolving her unfinished bonds with yokai. Many yokai initially mistook Takashi for Reiko due to their physical similarity, leading to conflicts and revelations about her past. Through Takashi's efforts to return the names in the Book, fragments of Reiko's life—including her loneliness, sporadic kindness, and unfulfilled promises—came to light, allowing Takashi to understand her isolation and the misunderstood intentions behind her actions.