Early LifeKurumi was once an ordinary human before becoming a Spirit. She grew up as the daughter of a rich family and was beloved by her parents. However, one day while returning home she was attacked by a monster, which probably a Spirit or some cosmic entity. She was saved when a girl wearing a dress made of light killed the monster. The girl introduced herself as Mio Takamiya and claimed herself to be an "ally of justice". She told Kurumi that the monster was called a Spirit and asked her if she wanted a power like hers to help her fight them. Wanting to protect her family and friends, Kurumi accepted and was given the Sephira Crystal containing Zafkiel. Afterwards, she bonded with it, unknowingly turning her into a Spirit herself.
Working together with Mio, Kurumi began hunting down Spirits. However, one day after killing a Spirit, Kurumi left Mio with the task of cleaning up the it's body. Kurumi then left, but took a wrong turn and returned to the scene. Now, instead of a monster, Kurumi found her best friend Sawa lying dead as Mio holding a red Sephira Crystal. Realizing Sawa was the Spirit and she had killed her, Kurumi started to inverse. However, before she completely transformed, she used her fourth bullet and reversed time. Mio then revealed the truth; she was having the Sephira Crystals bond with humans, turning them into monsters, and then kill the host in order to slowly purify the Crystals. Mio claimed that she intended to entrust the Sephira Crystals to someone before causing Kurumi to black out. Kurumi woke up in a giant crater after causing a spacequake with no recollection of who she was or where she came from. She eventually used her tenth bullet on herself to regain her memories. Afterwards, Kurumi made up her mind to use her twelfth bullet to travel back in time to kill Mio and prevent the existence of Spirits from ever happening.
At some point, using her ability to make clones of herself, Kurumi had created a massive information network. In order to gain enough time to use the Twelfth Bullet, she and her clones began to prey on humans, taking many lives in the process. However, the many lives she had taken attracted the attention of DEM , who sent Mana Takamiya after her. They fought many battles, during which Mana had always killed her. However, unbeknownst to Mana, she was just killing Kurumi's clones. Much later, <Phantom> appeared before Kurumi at some unknown point in time and told her about Shido Itsuka, along with the Spirit mana sealed inside his body. Having nothing to lose, Kurumi infiltrated Raizen High School through unknown means to confirm if this story was true.
The Black VisitorsKurumi transfers into Shido's class revealing herself to be a Spirit to get closer with Shido. During her time touring the school with Shido, who was oblivious of the fact that they'd been followed by Origami Tobiichi and Tohka Yatogami. At that time, Shido is attempting to seduce Kurumi, but she is also attempting to do the same thing to Shido. After school, Kurumi kills three delinquents that she met while walking along the streets and is then confronted by Mana Takamiya. Mana then runs into Shido later on that day. Taking her to his house, what Mana shares about herself leads to a quarrel between her and Kotori and when she was asked by Shido where she works, she ultimately leaves everyone with more questions than answers. During school the next day, Shido finds out that Kurumi was killed during her confrontation with Mana yet she appeared at school. Origami questions Kurumi in a secluded place and finds out that Kurumi is ultimately after Shido.
Triple Frenzy TuneShido invites Kurumi out but is forced on a polygamous date when Tohka seduces him and Origami forces her plans upon him for a date. Shido is unable to deny any of them as Tohka would be heartbroken, Origami would be suspicious and his plans to seal Kurumi depend on the date. With their dates occurring around the same time and their entertainment venues located rather close to each other, Ratatoskr assists Shido by teleporting him to each girl's location while monitoring their emotional levels.
After Shido leaves Kurumi, she finds some boys shooting at a stray cat and convinces them to let her play with them. Ratatoskr loses sight of Kurumi as Origami Tobiichi and Tohka Yatogami run into each other. After they both proclaim they are trying to find Shido for their date, Origami runs off concerned there may be another. Shido returns to look for Kurumi but finds blood, body parts, and Kurumi killing a man. As she holds Shido within her clutches, Mana Takamiya intervenes wearing her CR-Unit.
Ranging NightmareMana Takamiya kills Kurumi and explains everything to Shido. After that he is forced away, he then avoids Origami and Tohka after running into them because of seeing the death Kurumi caused. Tohka finds out the reason for Shido's behavior and continues her date with him to try to cheer him up. After gaining some insight to Kurumi, the next day Shido declares to Kurumi that he'll save her.
Kurumi goes to the school rooftop while encasing the school in her field which eats the time of people therefore making them faint to dissuade Shido from trying to save her. At the same time, Mana is currently confronting Kotori about Ratatoskr and attempting to use it as leverage against Kotori so she'll release Shido from the potential danger he's in. While Kurumi is threatening the school and the town, Origami and Tohka are engaging in battles with other copies of Kurumi. Shido convinces Kurumi only for her to be killed by another Kurumi. As Tohka and Origami show up, it's revealed that the hands in Kurumi's shadow are actually the hands of other copies of herself. Kurumi takes everyone as hostages and attempts to summon a spacequake only for it to be blown away. Kotori appears, explaining the cancelling of the spacequake while in her Spirit form. She has that time taken back her Spirit powers from Shido temporarily to fight Kurumi.
Spirit of FlameKurumi and Kotori Itsuka started fighting after a short conversation. Kurumi used one of her skills which is her Seventh Bullet, Zayin, to make Kotori stay still, creates copies of herself, and shoots Kotori a few times.
Kotori fell over after Kurumi's last shot but Kotori's healing skill enables her to not die and to heal from her wounds. Then, the shocked Kurumi sent her copies to kill Shido, but he was pushed away by Kotori and the copies were killed. In the heat of the fight, Kurumi used one of her skills to inhibit Kotori but it only worked for a while. Kotori used her weapon to shoot Kurumi's angel after the frightened Kurumi summoned her copies to shield her. Shido immediately stood in front of Kurumi to shield her, knowing Kotori is no longer herself when he tries to reason out to her not to kill Kurumi. After the shot was launched, Kotori suddenly regained consciousness and changed the direction of the shot hence saving Kurumi.
Things You Can't Give UpKurumi appears only at the end of the episode. After Kurumi have fled the battle against Kotori, she disappeared for 3 months in a row. However, Kurumi was still in Tenguu City. During these 3 months, she killed many people, stealing their lifespan and devouring them to mutton her hunger. Kurumi appeared in the last episode of Season 1 (Volume 4) as she had eaten else to sunset. In these events, a greater evil would appear. Kurumi also would have to unite with the heroes to fight against Isaac Westcott.
Kurumi and Shido decided the best course of action is to convince Miku to help leave them alone as they attempt to save Tohka from DEM. Along the way to Miku’s home, Kurumi relentlessly teases Shido about both “Shiori” and the size of Miku’s bra. However, the teasing was cut short, as they found a CD and a mysterious photo. Kurumi uses the Tenth Bullet -Yud- on the photo and CDs to peek into Miku's past, but the sound of Miku's Army Breaker Diva Gabriel interrupts Kurumi before she can explain to Shido about Miku's oddity.
Knowing that soon everyone in the area will be after them, Shido and Kurumi decide to confront Miku directly.
As Kurumi and Shido stand against thousands of people brainwashed by Miku, Kurumi summons her City of Devouring Time to take care of the mindless masses. However, Miku's Army Breaker Diva manages to give the people enough strength to overcome Kurumi's City of Devouring Time. Surprised, Miku boasts that her Army Breaker Diva isn't limited to enchanting people. Not one to be easily impressed, Kurumi and promptly summons her past selves (clones). The area immediately becomes engulfed in darkness as the battle begins.
With Yoshino and the Yamai twins busy fighting against the myriads of Kurumi selves, Shido is quickly taken to where Miku is by one of the past selves under the acceleration effect of the First Bullet Aleph. Kurumi drags Shido and Miku into her shadow so that the two can speak privately. However, the strain of fighting three Spirits and maintaining the space proves too much for Kurumi, who resorts to using the Second Bullet Bet to slow down time for their escape. Afterwards, Kurumi receives information about Tohka’s whereabouts from her other selves. She requests a pat on the head from Shido as a joke. Kurumi tells a flustered Shido that she is only joking and reveals Tohka's location; the DEM Industries Building No.1 of Japan Branch. However, as they arrive at DEM’s footsteps, Isaac Westcott actives spacequake alarm and send thousands of his of his soldier to kill Kurumi and her army. However, Mana appears and attacks Kurumi. Seeing her chance to move on her own, she decided to leave Shido to Mana.
During the aftermath of the battle, Kurumi remarked that she doesn't found her main target, the Second Spirit, the Spirit captured by DEM and knows the identity of the First Spirit.
Origami finds Kurumi and requests to be sent to the past with the power of Kurumi's Twelfth Bullet <Yud Bet>. Kurumi jokingly asks Origami if she wanted to go back so that she can see Shido when he was an innocent child, to which Origami replies that she wishes to go back five years ago to kill the Spirit that murdered her parents, thus changing history. Sympathizing with Origami's desire to alter the past, Kurumi eventually agrees to send Origami back in time. However, she tells Origami that she is using the former AST wizard as a test subject for the Twelfth Bullet and that she will use Origami's spiritual energy to power the time travel. Origami remains adamant and assures Kurumi that she is ready to pay the price as long as she is sent back before the incident which occurred five years ago. Then, Kurumi summons Zafkiel and shoots Origami with the Twelfth Bullet.
After seeing Origami tap into her inverse form, Kurumi takes the chance to talk to Shido. Kurumi tells him that everything he did will become meaningless once Origami went into her inverse form. She then proceeds to shoot Shido with the Ninth Bullet alongside the Twelfth Bullet, sending him to five years in the past to find out the reason for Origami