Yoru/History

Past
As a member of the powerful group of Devils known as the Four Horsemen. They, along with the Weapon Devils once waged a brutal battle against the Chainsaw Devil in Hell, but at the height of the battle, the Chainsaw Devil disappeared, making them unable to find him. During their battles, Chainsaw Man had eaten away parts of Yoru's body, weakening them. This, combined with the long absence of war among humankind, has greatly diminished Yoru's strength.

Justice Devil arc
Appearing on Earth, the War Devil took over the body of Asa Mitaka. After quickly disposing of the two humans who killed Mitaka as a warm-up, the War Devil vowed to make the notorious Chainsaw Man vomit back up Nuclear Weapons, which he previously ate. After the War Devil takes over Mitaka's body, Mitaka wakes up in her room and the War Devil explains their current situation. The War Devil took over half of Mitaka's brain, leaving Mitaka's consciousness intact alongside the War Devil's, and allowing the War Devil to appear as a hallucination to Mitaka. The War Devil explains that she requires Mitaka's help to function in human society and not be recognized as a Devil, all while threatening to kill Mitaka if she does not comply with her demands. When Mitaka returns to school, she does not want to ask Yoshida about Chainsaw Man, which makes the War Devil annoyed. The War Devil then takes control of Asa's body and confronts Yoshida herself, asking about Chainsaw Man and if Yoshida wants to be her boyfriend. After being told that Chainsaw Man might be in the Devil Hunter club and having her romantic advances turned down, the War Devil allows Mitaka to have control again and demands she join the Devil Hunter club. She also explains that anyone who "belongs" to her can be made into a weapon, hence her attempt to make Yoshida her boyfriend.

After Mitaka joins the Devil Hunter club, she befriends Yuko. The War Devil asks Mitaka to kill Yuko and turn her into a weapon, explaining that the more guilty Mitaka feels about killing someone, the better a weapon they'll make. Mitaka refuses and the War Devil relents for the time being. In order to switch topics, Mitaka diverts the War Devil's attention by demanding that the War Devil stop calling her "girl" and use her first name, Asa, instead. The War Devil responds that Mitaka also does not use her name, and Mitaka claims that "War Devil" is too awkward to use. After considering this, the War Devil says that Mitaka can instead call her "Yoru", and begins to call Mitaka "Asa".

When Yuko and Mitaka encounter the Bat Devil, Yoru realizes she cannot take control of Asa's body when Asa is stricken with fear. As she attempts to get herself and Asa out of the encounter alive, she once again demands that Asa murder Yuko, attempting to manipulate her by asserting that Asa only felt bad about killing Bucky because she was seen. When her attempt to convince Asa fails she becomes angry, and as Asa flees with Yuko in tow, the three are incidentally saved by Chainsaw Man.

Yoru is extremely agitated at being saved by Chainsaw Man and throws a childish temper tantrum, after which she admits to Asa that she hates Chainsaw Man because he ate a part of her and weakened her power. As a result of having part of herself eaten, war no longer happens, which leads to her growing even weaker in turn. Asa surmises that the War Devil is afraid of being forgotten.

When Denji proclaims that he is Chainsaw Man to Asa, Yoru takes it at face value and believes him. As a result, Asa calls her stupid, which makes Yoru angrily deny being stupid. Asa apologizes, but also tells Yoru that if she wants to find Chainsaw Man, she should come up with a plan herself, and offhandedly comments that he would probably come out if a Devil was drawn near the school, which Yoru thinks is a good idea.

When it is revealed Yuko made a contract with the Justice Devil and is going to attack the school tomorrow, Yoru sees it as an opportunity to lure out Chainsaw Man by battling Yuko at the school. When Asa protests, Yoru possesses her and won't give her body back until Yuko is dead. She takes the katana Asa received from the devil hunters club and meets Yuko in the school hallway the following morning. Yuko reads Yoru's mind and tries to convince her to let her kill Asa's bullies and they can fight afterwards, but Yoru isn't falling for it. When Yuko claims that Yoru can't beat her anyway, Yoru gleefully reminds Yuko that if she were to kill her, Asa would die too. Yuko admits she likes Asa, but dispises Yoru and transforms into a devil form to battle Yoru.

Yoru and Yuko fight fiercely, causing countless casualties in the process. Eventually, Yoru realizes that she can't beat Yuko and decides to leave the fight to Asa. When Asa uses her school uniform to create a sword, Yoru initially calls her weapon 'weak', but after Yuko is instantly cut to pieces with it, she calls it 'strong'.

After Yuko is revived by an unknown woman and Asa wakes up, Yoru immediately takes control of the body and confronts Chainsaw Man directly. She attempts to attack him but Denji is taken by the Octopus Devil out of sight. Believing he left because he saw her as weak, Yoru angrily exclaims for Chainsaw Man to show himself, as she will obliterate him the next time they meet.

The next day, Asa and Yoru head back to the school to recover the Super-Strong Uniform Sword, only to discover it was shattered during the battle. The pair are greeted by the Devil Hunters club, who inform Asa that they accept her into their ranks. Yoru takes over and asks if Chainsaw Man is among them. Sure enough, the student council president of the club pulls up his shirt and reveal a rip-cord sticking out of his chest, insinuating he is Chainsaw Man. After Yoru and Asa depart, Asa questions why Yoru didn't kill him then and there. Yoru admits that she needs a strong weapon to fight Chainsaw Man, and orders Asa to make a weapon even stronger than her Super-Strong Uniform Sword. Asa is irritated, but resolves to help Yoru defeat Chainsaw Man so Yoru will leave her body; and she is even willing to weaponize a person if necessary.

Yoru suggests that the powerful weapon they need lies between the lines at which Asa is willing to kill, and the point at which she refuses to kill; something akin to being between a cat and a criminal. Asa spots Denji picking up cigarette butts and recycling them into new cigarettes to sell to homeless people. Asa realizes Denji is between a cat and a criminal; someone who isn't bad enough to deserve death, but someone she is willing to kill as a necessary evil to defeat Chainsaw Man. Asa works up the courage to ask Denji out on a date, to which he reluctantly says "yes".