Yoru/History

History
Yoru had previously fought alongside the Four Horsemen and the Weapon Devils against Pochita in Hell. During their battles, Chainsaw Man had eaten away parts of Yoru's body, weakening her. It is also possible that this was the very same battle during which WW2 and the nuclear weapons were eaten.

Because of this there hasn't been a single war since "the world war" which further diminished her strength. Eventually Yoru got to the point where she was under real threat of being completely forgotten about which would mean permament death for her. This drove her to Earth where she would begin searching for a new body to possess in a last-ditch effort to save herself, as well as getting her revenge on Chainsaw Man.

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.

Dating Denji arc
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".

Asa and Denji's date to the aquarium goes poorly. Asa bores Denji with long-winded lectures on aquatic life, causing Denji to storm off. Yoru tells Asa to go after him, but she refuses, insisting she didn't do anything wrong. Suddenly, the girl who revived Yuko shows up at the aquarium. She introduces herself as Yoru's big sister the Famine Devil or "Fami". She agrees with Asa but elaborates that the true problem is that deep-down Asa cannot turn Denji into a weapon because she knows it is the wrong thing to do. Yoru, upon recognizing Fami asks Asa not to listen to her, but Fami snaps her fingers and transports Yoru "outside". She says that any human can abandon their morality when they are starving, and will create the right stage for Asa to turn Denji into a weapon. Denji returns and Asa realizes that the aquarium has been transformed into one long repeating hallway, with no sign of an exit.

After Asa destroys the aquarium by purchasing it and turning it into a weapon, Yoru is once again able to possess Asa. Yoru battles the Devil who has nowhere to hide after the building was destroyed, and kills the devil instantly with the power of the Aquarium Spear. After the battle, Denji walks Asa home and asks her to go on another date with him. Asa smiles and agrees to the date. 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."

Yoru fails to turn Denji into a weapon, while Denji interprets her actions as a farewell and does the same, touching Asa's head and saying: "Asa Spinal Cord Sword" before leaving. Yoru is puzzled by this, and Asa says that it must have been because he does not like her. Yoru tells Asa that their goal now is to take their time winning Denji's heart while also searching for Chainsaw Man. Asa questions her about Fami, but Yoru warns her to stay away from Fami before falling asleep.

For Asa and Denji's second date, they go to his home to watch movies. Yoru takes control of Asa's body and pounces on Denji, kissing him and once again attempting to turn him into a weapon. Suddenly Nayuta comes home from walking the dogs, calls Yoru a thief for kissing Denji, and links one of her chains through Yoru's skull. Using her powers, Nayuta has Asa and Yoru think they are a dog; and only agrees to turn her back if Denji stops being friendly with her. Denji reluctantly agrees, and Naytua erases Asa and Yoru's memories to make them think Denji stood them up. Back at the school, Asa tries to convince herself she is glad Denji stood her up but ultimately admits she is upset by it. Just then, Yoshida shows up and asks if Asa will keep him company.

Current arc
After Asa get's her feelings hurt by Yoshida, the two head back home. Asa falls into a depressed state and begins to break down. Yoru tries comforting her saying she's a bit too harsh on herself, without much effect. Asa ponders if she'd be better of dead (probably due to the Falling Devils power). Soon after a body falls next to them. Yoru swiftly takes over before Asa could get scared and tells her to stay back as "something's wrong". Yoru looks up to see dozens of people standing in their balconies staring down at her. They all begin to throw themselves to their deaths forming a huge pile of corpses in front of a very confused Yoru.

She summons a "Ruler Sword" and takes a stance as a mangled figure emerges from the pile of corpses. In response, Yoru drops her bag and runs away claiming she cannot possibly win this fight. While the figure reveals itself to be the Falling Devil, Yoru attempts to communicate with the now unresponsive Asa. She admits only Asa can create strong weapons and needs her to snap out of it.

Asa wakes up screaming in fear due to reliving a traumatic memory which removes the control of the body from Yoru. She finds herself falling upside down due to the Falling Devils power, but grabs onto a railing before she could fall. Unable to take back control, Yoru tells Asa not to be afraid. Asa continues having traumatic flashbacks. The force of the anti-gravity get's too strong and she loses her grip on the railing. While falling, Yoru instructs her to shout: "Nail Knife." Asa does so, and her fingernail rips itself off and transforms into a knife. The pain makes Asa forget about fear which lets Yoru regain control. She falls back to the ground as the gravity manipulation does not affect her. Yoru orders Asa to keep focusing on her pain and proceeds to speculate about the devil's identity before correctly assuming that it is the Falling Devil and that Fami's likely involved.

Yoru notices Asa's having a flashback again, so she promptly cuts her hand, believing it'll keep Asa from getting scared. Much to Yoru's surprise, this has the opposite effect, and Asa becomes afraid of Yoru instead. Before she loses control, she dashes under a building so as to not fall into Hell when gravity reverses on Asa.

The two then clash with each other, with Yoru being frustrated at Asa for not trusting her, while Asa retorts that she has no reason to as they don't even know each other, how Yoru always says disturbing stuff, takes over her body whenever and cuts her hand just now. Yoru tries to change the topic by blaming Asa's negative feelings on the Falling Devils' power. Frustrated at Yoru's lack of care and understanding, Asa responds by saying that since Yoru can read her mind, she should know what she's afraid of. Asa breaks down and explains that it's solitude and companionship. While she has trust issues, Asa gets lonely but fears how something always goes wrong when she tries to approach people and ends up alone all over again. Yoru seems at a loss for words.

Meanwhile the Falling Devil announces she plans to kill and serve both of them as her main course dish. Some time passes, with Asa and Yoru staying put until the Falling Devil finally arrives and prepares to kill them.