Reiko Natsume

Family: Takashi Natsume (grandson), Takashi's Mother (daughter)

Reiko Natsume is the maternal grandmother of Takashi Natsume, and the person whom Takashi inherited his strong spiritual power from.

She was despised by both humans and youkai. She was a bully towards humans and youkai alike, resulting in her being socially isolated. She defeated many Youkai and made them write out their names in the Yuujinchou (translated into Book of Friends), making them "servants" to her. However, there are times she is kind to humans but they don't return the gesture. For instance, she had taken a doll from a pot youkai, Kayatsubo to return to a human child but they told her that she could keep it, believing she stole it.

Initially, her actions were presented as a desire to have control over youkai, but it's revealed that they were done more out of loneliness and longing for companionship. Some of the youkai she "enslaved" or made plans with grew to despise her, while some youkai (like Santou) continued to wait for her return.

(Source: Natsume Yuujinchou wiki)