Not much is known about Izumi's history, but she is close friends with Yukari Sakuragi. She also has a troubled home life, having an older brother, Kazuma, who failed to find a job after leaving school, and now shuts himself up in his room. Her brother would often spend nights violently punching the walls of his room out of depression, placing severe strain on Izumi.

On the first day of Class 3-3, Izumi watched as fellow student Mei Misaki was chosen as the 'non-existent'. Mei was seemingly unaffected and calmly accepted the role. Having a troubled home life, seeing Mei so unaffected even when having such responsibility led Izumi to develop a deep resentment, bordering on outright hatred, but she did not reveal this to Mei.

Izumi debuts in Chapter Two, returning to school after an absence to find a new student in the class. Watching the student conversing with his fellow students from afar, Izumi asks Yukari about the student, and Yukari informs her that the student is Kōichi Sakakibara, the class' new transfer student, who arrived while Izumi was absent. Izumi chooses not to introduce herself to the new student.

It isn't until Chapter Four that Izumi makes herself known to Kōichi. In the corridor, Izumi overhears Kōichi asking Naoya Teshigawara and Tomohiko Kazami about the story of Misaki Yomiyama's death. Angry, Izumi asks Kōichi who told him the story and how much he knew. Though in actuality Mei told him, a startled Kōichi replied that he overheard the story from some other students, and only heard the part about Misaki dying. Izumi appears unconvinced, but at that moment, Ms. Mikami interrupts, ending the confrontation. Izumi then leaves and meets with Yukari, who was waiting at the end of the corridor.

Izumi is present during the exams at the end of May and watches as Kōichi finishes early and is excused from the classroom along with Mei. Minutes later, a fellow teacher rushes into the classroom and informs Yukari that her mother had been in an accident. Izumi presumably watches as her best friend rushes out of the classroom. As she makes her way down the stairs, Yukari slips and her umbrella impales her through the throat, killing her instantly.

The incident leaves the class terrified that the deaths of Yukari and her mother meant the calamity had begun, and Izumi, in particular, was grief-stricken at the loss of her friend. Near the beginning of Chapter 5, Izumi attends their funeral and weeps at the sight of the dual coffins. At some point, Izumi learns that Takako Sugiura saw Yukari through the classroom window, seconds before her death. Yukari had initially made her way to the east staircase, which was closer, but encountered Mei and Kōichi. Shaken at the sight of Kōichi conversing with the non-existent, Yukari changed direction and rushed to the west staircase, where she met her demise. Izumi blames Mei and Kōichi for Yukari's death, Mei in particular. In Chapter 6, Izumi enters the classroom later than usual and ignores Kazami when he comments on this. Brushing past Kōichi, Izumi coldly glares at him and tells him that she will not forgive him.

In Chapter 7, the day after Takeru Mizuno's sister, Sanae, is killed in an elevator accident, Izumi, Naoya, Takako, and another student spot Kōichi discussing Sanae's death with Mei, treating her like she existed yet again. Growing more and more concerned that Kōichi's actions are endangering the other 3-3 students, Izumi and her friends decide to hold a class meeting to deal with the situation. While Kōichi is called to the student council room for police questioning concerning the circumstances behind Sanae's death, the entire class is called to a meeting room. During the meeting, Izumi is made the new female representative, replacing the deceased Yukari, and a new countermeasure is made: they will turn Kōichi into a non-existent along with Mei, hoping to undo the damage he had presumably caused by fraternizing with her.

Izumi and the rest of the class put this countermeasure into effect the next morning, after Ikuo Takabayashi died at home due to illness. At the beginning of the class, she, Shoji Kubodera, and Tomohiko oversee the farewell ceremony for Ikuo, and urge the class to keep the new countermeasure in place. When Kōichi, confused as to why no one was acknowledging him, confronts Naoya, he cries out in shock. Quickly intervening, Izumi approaches and tells Naoya not to talk to himself.

Izumi and the rest of the class continue to uphold the new countermeasure until Chapter 11. One morning, Mr. Kubodera enters the classroom, appearing slightly unstable. Unknown to the students at the time, he had become overcome with despair and had killed his bedridden mother, Tokue, hours before. Standing at his desk, Mr. Kubodera apologizes to the students for being unable to teach them any longer, and suddenly begins twitching. Pulling a knife from his bag, Mr. Kubodera tells the students that the calamity is their problem from now on and proceeds to stab himself in the throat. The horrified students flee the classroom, and those remaining are ordered out by the school librarian, Tatsuji Chibiki, when he discovers the scene.

With it now obvious that making two students non-existent had failed to stop the calamity, a traumatized and enraged Izumi tearfully confronts Kōichi and Mei outside the classroom, telling them that all the deaths so far were their fault for consorting with each other. Izumi continues to scream that it was all their fault as the chapter ends.

After Mr. Kubodera's death, the original classroom is closed off and Class 3-3 is moved to a new room. Once the new countermeasure proved to be a failure, the class began acknowledging Kōichi and Mei's existence again. After her outburst towards the two students, Izumi kept her distance from them. Eventually, Ms. Mikami informs the class that in a few weeks, they will go on a class trip to the Yomiyama Shrine, hoping that praying to the shrine will stop the calamity.

At home, the situation with Kazuma grew steadily worse: her brother's habit of punching the wall at night continues to distress Izumi, who would cover her ears every time in an attempt to shut out the noise. With the stress caused by Kazuma and the threat of the calamity looming over her, Izumi begins to grow terrified that she might be the next to die. One evening, Izumi lies in her bed, gazing at a picture of her and Yukari, when she hears a crash outside. When she goes out to investigate, she finds that Kazuma had dropped a plate while getting dinner. He glances at Izumi before rushing back into his room, leaving their mother to clean the mess.

Later that night, after another round of punching the walls that drives Izumi to tears, her bedroom becomes too hot to sleep, and she steps out into the corridor. She notices that Kazuma's bedroom door is open. Finding this out of place, she investigates and discovers that her brother had hung himself. Distraught at the sight, Izumi breaks down screaming. Knowing that there was no one she could count on to save her, Izumi decides at that moment that from now on, she would count on herself.

On the 8th of October, Izumi is one of the few students attending the class trip. On the hike up to the Sakitani Memorial Hall, where the class would be staying for the next couple of days, Naoya expresses his condolences to Izumi for Kazuma's death. Izumi merely replies that it couldn't have been helped. When they reach the hall, Yūya Mochizuki suggests taking a commemorative photo, which the class accepts. Izumi does not participate and stands to the side, glaring hatefully at Mei all the while.

It is decided that the class will travel to the shrine tomorrow, and in the meantime, the students are to wait in their rooms until dinner. While walking through the corridor, Izumi spots Mei going into Kōichi's room. Intrigued, she listens at their door and finds that the pair is listening to a tape that Katsumi Matsunaga, a student from fifteen years ago, had made. What she learns amazes her: fifteen years ago, the class had also gone on a class trip to the shrine. Praying to the shrine failed to stop the calamity, and two students died that same night. Eventually, Katsumi and another student got into a fight over the deaths, which ended with Katsumi killing the student. The student disappeared without a trace the next morning, and the other students no longer remembered him ever existing. Katsumi eventually figured out the truth: the student he had killed had been the Extra, and killing him had stopped the calamity for that year. Izumi is shaking by the time Katsumi gives the following advice: kill the Extra and 'send the dead back to death.'

That afternoon in the dining hall, once everyone had eaten their meals, Izumi stood up to give a speech. Once Ms. Mikami gives permission, Izumi openly confronts Mei and Kōichi, claiming that they were responsible for the misfortune the class has experienced. She places most of the blame on Mei in particular, for not properly playing the role of the non-existent, and expresses anger that after she stopped being the non-existent, she apparently continued acting as if nothing was wrong. Izumi demands that Mei apologize for her actions. Though Mei declares that apologizing would do nothing to help the class, she complies. A distraught Kōichi angrily defends Mei and claims that it was his fault, and an argument ensues as Yūya and Naoya also step in, declaring that stopping the calamity was more important. The argument is cut short when Kōichi collapses from his illness and is taken back to his room.

After dinner, Izumi becomes convinced that Mei is the extra one, due to her emotionless personality, which Izumi likens to a doll, and makes plans to kill her. When Kōichi and Naoya leave the building to find an injured Tomohiko, leaving Mei alone, Izumi seizes her chance. She lures Mei to the kitchen, which hosts a variety of potential murder weapons, and reveals that she knows about the tape and even questioned Naoya about its contents after dinner. Grabbing an ice pick, Izumi swings at Mei, but only manages to scratch her face and knock her to the floor. Izumi declares that killing Mei would stop the calamity and asks if she agrees. When Mei replies that she is not the dead one, an enraged Izumi grabs her by the hair and screams at her to shut up. Utterly convinced that Mei is the extra, Izumi throws Mei back to the floor and pins her, and in her rage, reveals how much she has hated her since the beginning of the school year. She shouts that she has always hated that Mei is so emotionless, even when she has the world on her shoulders, as if she were just a doll. Izumi's words cause Mei to recall memories of how her adoptive mother always treated her like another of her dolls, and she shows emotion to Izumi for the first time, declaring that she is not a doll and she is not dead. Izumi screams at Mei to shut up and thrusts the ice pick into her left eye.

Believing that she had killed Mei and would no longer need to fear dying, Izumi drops the ice pick and laughs in satisfaction. Suddenly, Mei moans and tells her again that she isn't dead. When a shocked Izumi looks closer, she sees that the eye she stabbed is made of glass. As she takes this in, a stroke of lightning strikes the building, shaking the kitchen and knocking Izumi to the floor. As fire begins to consume the building, Mei gets up and tells Izumi to leave, lest she becomes a victim of August. Feeling no ill will towards Izumi for what she had tried to do, before she leaves, Mei tells her once more that she is not dead. She adds that Izumi isn't dead either: she is still alive. Mei's words greatly affect Izumi, and when Mr. Chibiki finds her in the kitchen minutes later and carries her to safety, she mourns for Yukari and Kazuma.

Ultimately, Izumi survives the calamity: while she is being rescued by Mr. Chibiki, Mei and Kōichi find the true Extra, Ms. Mikami, and kill her, ending the calamity for 1998.

After the disastrous class trip, Izumi is sent to the hospital with minor injuries, and her hair is cut into a bob after it was burned in the fire. While she is reading in bed, Izumi is surprised when a recovered Tomohiko shoves a reluctant Naoya into the room. Having had a crush on Izumi for some time, Naoya nervously greets her and comments that she cut her hair. Finding his comment oddly amusing, Izumi giggles, showing happiness for the first time in the series, and tells him that her hair was burned.

Being the first adaptation of the novel, the manga's version of Izumi is the one upon which all future iterations of the character are based (Izumi was a minor, supporting character in the novel, but has become a major character and most often a foil for Mei in all other versions). In the manga, however, her role at the beginning was significantly smaller than in the anime: she was not present when Tomohiko and Yukari met Kōichi at the hospital, for example, and did not shake his hand. She is also much closer to Yukari in the manga (the two were at best merely acquaintances in the anime, as Izumi was much closer to Takako, who had a much larger role in that adaptation). Though Kōichi comments at one point that Izumi is often at the center of a huge group of friends, the manga version of her character is much more withdrawn and alone, unwilling to seek help from others. She is also much more hostile toward Kōichi in the manga; in the anime, the two were fairly close friends by the end.