"Sis, what should I do?"

- Reiko in the manga.

Reiko Mikami was Kōichi Sakakibara's idolized aunt, whom he lived with after moving to Yomiyama. She was at Yomiyama North Middle School along with Katsumi Matsunaga and Ryoko Akiyama in 9th Grade Class 3.

Reiko looked young for her age and was seen wearing a plain yellow top with long green pants at home. She had long, straight brown hair and brown eyes. She also had her hair tied in a ponytail, wore a pink clip, and spectacles with squared lenses.

Reiko gave Kōichi lots of advice on how to get by living in Yomiyama. Many times, she stated that she hated the family pet bird, Rei. She seemed to get very upset whenever someone talked about the incident 15 years ago, when she was in the 9th grade in Class 3-3. Also, at that time, she didn't believe in the curse until her best friend, Ryōko Akiyama, died. When she saw that day by day, more classmates and their relatives were dying, she wanted to find out how to stop the calamity, asking Mr. Chibiki about it. Reiko claimed that her eldest sister, Ritsuko, was the woman she admired the most and also her idol, and that she was very happy when she came to Yomiyama. She loved her very much, and Reiko frequently visited her at the hospital after she gave birth to Kōichi. Also, before Kōichi came to live with her, she claimed that she would protect him, as Ritsuko said that she would protect him shortly before she died. At the end of the manga, Tamie Mikami said after her death that she would offer herself to take care of Kōichi, as Yōsuke Sakakibara can't do it, even if she had met Kōichi a few times.

Reiko was fond of alcohol and often ended her days with a drink. In the manga, she seemed to be a fan of Sapporo, a popular domestic Japanese beer, and in the anime, she was seen fixing herself a "Red Eye."

In Another 0 Chapters, Reiko is seen in her younger days, when she was a student in the 9th-grade Class 3-3. At that time, she claimed that she didn't believe in the calamity. Also, she often visited her eldest sister, Ritsuko, who was in the hospital shortly after Kōichi's birth. After her best friend Ryoko dies, she tries to find a way to stop the calamity.

In Episode 1, Reiko first appears at the hospital when she visits Kōichi, who was recovering from a collapsed lung. While there, she tells him information about how to live in Yomiyama and which school he is going to attend. When Kōichi becomes suspicious about his class' curse, he asks her about it, but she yells and says that she can't help him. She is Kōichi's aunt from Ritsuko's side and Class 3-3's deputy homeroom teacher. She gives her nephew advice about Yomiyama Middle School through a series of "rules". One of them is to separate their private life from their school life; thus, Kōichi addresses her as aunt or Reiko at home and Ms. Mikami at school.

In "Hair Stand", Reiko takes Kōichi and the others to the beach resort to look for Katsumi Matsunaga. They had fun on the beach and at the end of the episode, they witness Junta Nakao's death on shore.

In "Glass Eye", while Kōichi and Mei Misaki were looking at the pictures inside the box, they found a picture of Reiko painting. Mei then asked Kōichi if Reiko paints at home. He told her that his aunt does, and she considers painting her real job. In the anime, no episodes showed Reiko painting. There is an episode showing Ms. Mikami supervising an art class at school.

In "Stand by Oneself", the connection between Reiko and Ms. Mikami is revealed. Also, it is revealed that she is the Extra this year. Reiko was stabbed by an unknown assailant and thrown into the river in October of 1996, where she drowned. Mei saw her death, but didn't remember due to the curse altering her memories. She had known that Ms. Mikami was the Dead One, but had said nothing because, so far as she knew, nothing could be done about it. After Reiko was killed a second time in August 1998, nobody remembers her presence in 1998 or sees her in the photos made on the class trip, except Kōichi and Mei, who were present during her second death.

Kōichi and Reiko were very close and often spoke. She often gives her nephew advice while at home; their relationship is very different at school. Sometimes in the manga, they have a closer relationship. Kōichi was shocked upon realizing that she was the "Extra," and before killing her himself (instead of Mei), he said "Goodbye mother," suggesting that he saw her as a mother figure rather than an aunt. In the manga, he sometimes attaches "-nee" to her name, a shortened form of nee-san, or "big sister." They are seen to have a stable relationship.

Reiko and Ryōko were close friends. Reiko was shocked after her death, believing that the "Calamity" had begun.

Ritsuko is Reiko's sister by eleven years. They were friends, and Reiko admired and idolized her older sister and was very affectionate towards her son, as seen in Chapter 0.

Not much is dwelt upon between them. In the climax, Reiko was willing to shield Mei from an angry mob when they thought she was the extra. When it turned out that Reiko was the Extra, Mei attempted to kill her to spare Kōichi the heartache. Reiko herself was obviously terrified and confused, going as far as calling Mei a liar when the accusation flew. Mei is the first to realize that Reiko is the dead one.

In Episode 8, Reiko, along with Kōichi, Izumi Akazawa, Naoya Teshigawara, Yūya Mochizuki, Takako Sugiura, and Junta Nakao, met Katsumi on the beach to try to get information involving the Curse. Katsumi and Reiko were in the same class 15 years ago and seem to have a comical and good relationship, as Katsumi says Reiko was a "shrimp" and Reiko says he is an "old geezer". They reconcile and talk about what they've been up to, with Katsumi bringing up another point on Reiko's death, as he saw and heard something about her recently.