Reiko physically resembles Takashi Natsume's (Reiko's grandson) characteristics through her long, dusty-blonde hair, amber eyes and she usually carries a slight smile, just like Takashi. It is because of their similar appearances that youkai often mistake Takashi for Reiko. She has been noted to be quite beautiful by both youkai and humans.  Her hair goes to her bust-level, and usually, it seems to be unkempt.
In the anime and manga, she's usually seen only in flashbacks wearing her school uniform, which is sometimes torn. She once had a bruise on her cheek because human children threw rocks at her.
Reiko 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 over the course of the series, it's revealed that they were done more out of loneliness and longing for companionship. She died young with no human willing to remember her with many youkai mistaking her grandson for her.
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.
Reiko had strong spiritual power, enabling her to see youkai. Due to this ability, she was shunned by humans, thus, she found partial comfort amongst youkai. She would take out her frustration on the youkai by bullying, teasing, and challenging them to duels.
From then on, until her death, she would be with youkai more often than humans. She would challenge youkai to a duel each time she encountered one and then have them write their name down in the Book of Friends if they had lost.
She had a good relationship with some youkai but others were forgotten. She would constantly forget about the youkai whose names were in the Book of Friends or the plans she made with youkai. One definite exception is that she returned to an Acorn-Eating Youkai to give him mochi.
This led to her grandson having to tie up all the loose ends that she left for him. She was well-known for her creation of the Book of Friends which led to many youkai wanting the book for themselves, even attacking Takashi Natsume to steal it.
The details of her death are unknown and haven't been mentioned yet in the series. However, Takashi said (in a conversation with Nanase) that Reiko died under a tree somewhere.
- Main article: Madara
Reiko's relationship with Madara is not yet revealed but it's hinted as intimate. Madara can imitate Reiko in terms of her appearance, her habits and manners. He personally states that other than Takashi Natsume, she is the only person Madara has "taken a good look upon", which indicates that they had a close relationship.
- Main article: Souko
- Main article: Shigeru Fujiwara
Reiko first met Shigeru (when he was a child) when he was struggling to get a ball out of a tree as a youkai was holding it. Shigeru thanked Reiko for helping him and some boys came along which threw a rock at her and Reiko grabbed a stick that Shigeru was holding and hit the rock back at them likewise to a baseball bat hitting a ball, resulting in Shigeru's amazement at the feat. Shigeru noticed that whenever he saw Reiko, she was always alone and despite her ‘odd and scary’ behaviour, Shigeru approached her and began to talk to her. One day, Shigeru's family was experiencing problems in their home (due to a youkai named Karime) but Shigeru thought it was a coincidence rather than a curse which everyone else thought it was.
Reiko asked Shigeru if she should come to his house and he accepted, which led to her going to Shigeru's home one day. When Shigeru went to make refreshments, he heard a noise upstairs and then he went to the room that Reiko was in to discover that it was destroyed.
Shigeru fell asleep at one point and when he woke up, Reiko was gone and Shigeru was scolded (since he took the blame). After that, they never spoke again and Shigeru forgot about Reiko, only remembering her actions, the sight of the destroyed room and not her name.
Powers & Abilities
- Reiko had unusually strong spiritual power, which her grandson Takashi inherited. Her strong spiritual powers allowed her to see youkai.
- Her great power enabled her to easily beat even the strongest youkai in matches. She had forced them to write their names in the Yuujinchou after their defeat. By writing their names, the youkai are bound to the Yuujinchou. The Yuujinchou contains the names of both weak and powerful youkai, making it a common target for power-seeking youkai. This is because the youkai in the Yuujinchou have to obey their master, regardless of what it is and could also be killed easily using the Yuujinchou.
- Reiko is often shown to be very knowledgeable of sealing and exorcism techniques which she probably extracted from the youkais she had beaten.
- Reiko is often seen wielding a baseball bat plastered with talisman to defend herself against youkai.
- Reiko's surname Natsume means "summer eye" in Japanese.
- Reiko has bad table manners. In one case, her table manners resulted in a piece of rice sticking Tsuyukami's and Susugi's pages together in the Book of Friends.
- Reiko was also very forgetful which is seen throughout the series as she forgot to meet up with youkai who were anticipating her return.
- It has been mentioned by Shuuichi Natori in Chapter 62 that making a contract using a youkai's real name is forbidden. Therefore, the Yuujinchou's existence is actually forbidden and it is unclear whether or not Reiko was actually aware of this fact.
- Reiko continuously challenged Madara to duels, however, he had continuously refused her. This resulted in their relationship being one of friends but she had suddenly stopped coming at one point.