Yoru is the villainous deuteragonist of the Academy Saga. She is the War Devil who embodies the fear of war and conflict. She is a member of the Four Horsemen, who now occupies Asa Mitaka's body as a Fiend. Following the battle with the Four Horsemen and Weapon Devils, Yoru was also reduced to a “near-death state”, due to Pochita consuming most of her body. In this state, she appears to have a bird-like body, similar to a dark colored owl or potoo. While her bird form is similar to Pochita's dog form, her true form is still unknown. After taking over Asa's body and occupying half of her brain, the War Devil took on her appearance, but with added facial scars across her the center of her face and left cheek, which are in the same location where Mitaka's face was slashed by the Class President. In this form, Yoru's eyes have multiple rings, similar to Makima, who was another Horseman. Her hair is also usually worn loose rather than in twin tails like Mitaka. Yoru always wears the same clothes as Asa, most commonly a school uniform. Yoru is first shown to be very confident and prideful, treating her opponents as mere warm-ups and confidently vowing to take back nuclear weapons from Chainsaw Man. This might be a sign of her over-confidence as she lost to him previously while at full strength and is significantly weaker now. However she is aware of her limitations and will back down from a fight if she believes the odds are against her. Yoru holds little sentimental value or attachments to humans, and is mostly uncaring of Mitaka's feelings, as evidenced by her blatant disregard for Mitaka's feeling of friendship for Yuko, repeatedly asking Mitaka to kill Yuko to use as a weapon or when she goes as far as to physically hurt Asa herself to forcefully regain control albeit with the intention of saving them both. She's been shown to be very cold and logical which clashes with Asa's highly emotional personality. Since she's completely unaffected by the Falling Devil, that means she easily gets over highly traumatic events such as being almost killed by Pochita and losing her power. Due to her predisposed nature as the War Devil, she keeps getting into conflicts with everyone around her, sometimes intentionally sometimes not. Ironically these conflicts usually serve to hurt her. Unlike her fellow Horseman Makima, who was almost always calm and composed, Yoru is more flawed, immature and naïve. She is prone to losing her temper, and became embarrassed and angry at being rescued unintentionally by her archenemy, Chainsaw Man. In her anger, she sometimes throws childish tantrums by punching and screaming into the pillows on Asa's bed. She is very insecure and gets royally offended whenever her strength or intellect is questioned. Yoru is also worried about losing her power and being forgotten, showing a more sensitive side. Adding to that, she seems to show signs of sympathy, at least towards Asa. She does not have a good understanding of human society or relationships and takes things at face value, is easy to provoke and often threatens Mitaka with death. Though over time she grows closer to her and seems to understand and adapt to human society more, albeit she's still quite inept and will often unintentionally worsen her relationship with Asa because of that. Despite her apparent social-ineptitude she is very cunning in dangerous situations such as when fighting Devils and is easily able to keep a cool head unlike Asa. Though in the most extreme of circumstances Yoru will panic and scream out in fright, such as when Asa was caught by the Falling Devil.