Asa Mitaka/History

Justice Devil arc
Asa is a student attending class at Fourth East High School. She is distant and unsociable to her classmates, many of whom don't even know her name. Asa is further alienated by her class when she accidently trips and kills Bucky the Chicken Devil, a beloved class pet. In an effort to cheer her up, the teacher and class president invite Asa to make amends over Bucky's grave; only for it to be revealed the class president and the teacher have been having an affair. The class president attempts to kill Asa using the powers she obtained from her contract with the Justice Devil. In her dying moments, Asa is greeted by Yoru the War Devil, who will save Asa's life if she shares her body. Asa is resurrected as the War Fiend, Yoru takes over and quickly dispatches her teacher and classmate. Yoru reveals to Asa that Chainsaw Man attends her school, and that her mission is to defeat him. Being a devil, Yoru doesn't know much of human society so she kept Asa alive by leaving half of her brain intact, the two now share one body. Yoru pushes Asa to join the school's Devil Hunters club in an effort to find Chainsaw Man's true identity, but Asa is bullied and embarrassed by her classmates who still blame her for Bucky's death. She is cheered up by a fellow club member, Yuko, and Asa soon looses focus on finding Chainsaw Man instead opting to enjoy the time she is spending with Yuko. While the two are out patrolling for devils, they are attacked by the Bat Devil. Yoru encourages Asa to kill Yuko and turn her into a weapon so she can fight the Bat Devil, but Asa is unwilling to harm her first friend even if it puts her own life in danger. Asa and Yuko are inadvertently saved by Chainsaw Man, who kills the Bat Devil by accident while fighting the Cockroach Devil. The next day Yoshida introduces Asa to Denji and reveals he's a Chainsaw Man fan. Denji and Asa get into an argument after Asa admits she did not use to like Chainsaw Man. As their argument escalates, Denji reveals he is Chainsaw Man. Asa doesn't believe him, calls him a loser, and dismisses his claim as nothing more then ramblings from a Chainsaw Man fan.

Asa brushes off the Devil Hunters club to visit Yuko, who was injured during the Bat Devil attack. Yuko admits she doesn't have many friends, and that Asa is the first person to visit her after her injury. In an effort to strengthen their bond, Yuko and Asa reveal their deepest darkest secrets to each other; Asa recounts the time she peed herself in grade school, and Yuko admits she killed her neighbor the other day. When Asa asks why, Yuko says she had discovered her neighbor had been gambling his welfare money away, a fact she learned after making a contract with the Justice Devil and gaining the ability to read minds. Yuko goes on to reveal she knows Asa killed her teacher and class president, and she knowns about Yoru; but she'll keep Asa's secret. Yuko wants to save Asa like Asa saved her, and tells Asa not to come to school tomorrow as she will be taking care of her bullying problem; permanently.

Asa walks home, contemplating what Yuko said. She realizes Yuko is going to kill the bullies at school and wants to go back and talk her out of it, but Yoru wants to battle Yuko and use their fight to lure out Chainsaw Man. When Asa protests, Yoru possesses her and wont give her body back until Yuko is dead. The next morning at school, Yoru confronts Yuko with a katana in the hallway. Yuko reads Yoru's mind and learns of her plan, and tries to convince Yoru that she can't win in a confrontation with her. Yoru only smiles and reminds Yuko that killing her would kill Asa as well. Reluctantly, Yuko transforms into a devil form to engage with Yoru.

Yoru's attack's prove unsuccessful and several students are killed. Yoru instead chooses to release control of their body back to Asa in an effort to defeat Yuko. Asa uses the War Devil's abilities to turn her school uniform into a weapon, and warns her Yuko to stop her assault. Yuko doesn't take Asa's threat seriously and goes to kill the last of Asa's bullies. Asa retaliates by striking Yuko with her Uniform Sword and, much to everyone surprise, Yuko is sliced to ribbons from a single strike of the sword. When Yoru questions why the Uniform Sword weapon was so powerful, Asa remarks that the uniform was bought by her deceased mother and the guild she felt in turning it into a weapon caused the sword to be especially strong. As Yuko lays dying, a mysterious girl appears and asks Asa if she wishes to save Yuko; Asa's answer is "yes". The mysterious girl calls Asa her "little sister" and is shown to possesses ringed eyes similar to Yoru's and Makima's. Suddenly, Yuko is revived as a giant version of her devil form's head explodes from the side of the school building. Asa passes out and the girl leaves.

Early the next morning, the doorbell rings. It is Yuko, who says she has been turned into a Devil and may try to eat Asa, so she should not open the door. Asa opens the door anyways and sees Yuko's horribly mutated new form. Yuko came to tell Asa she is leaving to be healed, and only stopped by to say goodbye to her. As a parting gift Asa gives Yuko her shoes, and as Yuko leaves Asa goes back to sleep.

Current Arc
Later, Asa and Yoru visit the partially destroyed school and find their Unform Sword in shambles. Yoru explains that while the uniform sword was strong, it was also fragile. Asa asks Yoru if she knows the woman who revived Yuko, but Yoru says no and passes it off as the girl being a madwoman. Haruka Iseumi, the Devil Hunter Club's president, introduces himself to Asa. He tells Asa that she is accepted into the club. Haruka reveals a pull-cord sticking out of his chest. Asa is skeptical that he is actually Chainsaw Man. Yoru asks Asa to create a weapon that is even stronger than her uniform sword. Asa says she is now willing to turn a human into a weapon if it means that Yoru will leave her body so she won't ruin other people's lives as a result of Yoru's presence. Yoru suggests that the most powerful weapon would be the line in the middle of killing a cat and killing a criminal. Asa spots Denji, who is picking used cigarette butts off the ground. She realizes Denji is right in the middle between a criminal and a cat, not so bad that he deserves to die but the necessary evil to defeat Chainsaw Man. Asa confronts Denji, but Denji is aggressive to her because of how she treated him before. Asa asks Denji on a date, and he reluctantly says yes.

Asa and Denji's date proves disastrous, as Asa bores Denji with facts on sea urchins and sea stars during their visit to the aquarium. When Denji suggests going to see the penguins, Asa asks him to shut up and not to think about a thing, insisting they stick to the schedule she planed out. This infuriates Denji and he leaves. When Yoru asks Asa to go after him she refuses, insisting she didn't do anything wrong.

Suddenly, the girl who revived Yuko shows up and properly introduces herself as Yoru's big sister, the Famine Devil or "Fami", as she prefers to be called. Fami agrees with Asa, but elaborates the problem does not lie in the date, but that Asa is still unwilling to kill Denji because deep-down she knows it is the wrong thing to do. Yoru tells Asa not to listen, but before she can continue, Fami snaps her fingers and transports Yoru away temporarily. She tells Asa than any human will shed their morality when they are starving, and she won't release Asa until she is able to turn Denji into a weapon. Denji returns and Asa notices the aquarium has become one long repeating hallway. Denji remarks that that "pain in the butt" is back from hell, insinuating the return of the eternity devil.

As Asa and Denji traverse the hallway, they encounter other members of their schools Devil Hunter Club as well as Yoshida. They form a group to try and find a way out, but to no avail. While the group finds food and water in the aquarium staff lounge, Asa catches Denji stealing money from a bag and berates him for not helping the group. Denji retorts by asking what contribution she has made since they have been trapped, and as Denji storms off, Asa remembers she still has her mothers cell phone much to the excitement of the other students. While looking for a signal to call for help, Asa trips and breaks her phone. The club president sighs and exclaims his disappointment in Asa, saying that she has done noting but complain and been a loner since they have been trapped, and she cannot even come through in a clutch. Embarrassed and ashamed, Asa sits alone in the hallway.

Days pass in the aquarium. Denji checks up on Asa, who apologizes and blames herself for the situation they are in. She admits that despite how bad things are, she remains unable to turn Denji into a weapon; not because it goes against her morals, but because she's terrified of making the wrong choice. Denji sits down with Asa, smiles, and asks if's she's ever eaten starfish.

Asa and Denji bond as they eat starfish together. Asa comes up with a plan and asks Denji to help her collect money he has been finding in the aquarium. Eventually, the two manage to collect one million yen, which Asa uses to "buy" the aquarium and turns it into a weapon so the group can escape. After the aquarium gets turned into a spear, the Eternity Devil appears since it no longer has a building to hide in. Yoru is able to posses Asa once more and fights the Eternity Devil. Due to the guild Asa felt from ruining the aquarium, the spear is strong enough to kill the Eternity Devil with a single strike.

After the battle, Denji walks Asa back to her place. Denji asks Asa to go on another date with him; Asa smiles and agrees too. Yoru comments on Asa's affection for Denji, which Yoru is able to feel due to sharing a brain with Asa. Despite this, Yoru decides to proceed with the plan and possesses Asa, touches Denji's head, and calls out "Denji Spinal Cord Sword."