Asa/Yoru

Yoru's abilities revolve around his identity as the Demon of War, allowing him to transform his surroundings into lethal weaponry. His main powers are:
Weapon Creation (Object Conversion): Yoru can convert anything she considers "hers" into a weapon. To activate it, she must touch the object or person and say the name of the weapon she wishes to create (e.g., "Tanaka's Spinal Marrow Sword").
Rule of Ownership: The effectiveness of power depends on Yoru or Asa's perception of ownership. She does not need to be the legal owner; it is enough that she believes something belongs to her, whether by having bought it, having defeated an opponent, or through an emotional connection.
The Guilt Factor: The power of a weapon is directly proportional to the guilt the user experiences when creating it. Because Yoru is a demon and feels little empathy, he relies on Asa Mitaka's strong moral compass and insecurities to forge the most devastating weapons.
Demon Attributes: Possesses superhuman strength, agility, and reflexes, far superior to those of an average human. Furthermore, can regenerate by consuming blood, a standard ability of demons.
Empowerment Through Fear: Like all demons, her power fluctuates according to humanity's fear of war. In recent events in the manga, following the resurgence of certain global fears, she has demonstrated the ability to create weapons of massive scale and even manifest war gauntlets based on other demons, such as the Weapons Demon or the Tank Demon. As Yoru is one of the Four Horsemen of the Apocalypse, she is extremely powerful.

Asa Mitaka is the complete opposite of Yoru: an introverted, socially awkward girl with a highly complex inner life. Her personality is defined by the following traits:
Socially Isolated and Cynical: Due to past traumas, Asa is a "loner" by choice (or defense mechanism). Initially, she displays an air of intellectual superiority toward her classmates to mask her envy and loneliness, as seen in her early monologues in the Chainsaw Man manga.
Morally Strict and Anxious: She is extremely rigid about rules and what is "right." She tends to overanalyze everything, which often leads to analysis paralysis or panic attacks.
Deep Insecurity: Despite her distant facade, she desperately wants to be accepted and loved. Her low self-esteem makes her vulnerable, but it's also what allows her to create powerful weapons, since their strength depends on the guilt she experiences.
Clumsy but Resilient: He has almost comical bad luck (frequently tripping or putting his foot in it at critical moments), but possesses an iron will to survive despite his constant desire to give up.
Honest to the core: Unlike Yoru, Asa is unable to hide her feelings for long. Her internal struggle between wanting to be a "good person" and having to kill to survive is the driving force behind her development.
After sharing half of Asa's brain, Yoru can feel the same feelings as Asa and feel the same things.

Yoru's personality is radically opposite to Asa's, defining her nature as the Demon of War. She is characterized by being:
Arrogant and Proud: Yoru possesses an immense ego. She sees herself as a superior being.
Impulsive and "Dim" (in the style of Power): Unlike the cold and calculating Makima, Yoru often acts impulsively and sometimes childishly. Fans often describe her as someone who "thinks she's a mastermind but often messes things up" due to her lack of understanding of human society.
Ruthless and pragmatic: She has no qualms about sacrificing human lives to turn them into powerful weapons. Her logic is purely military: she only cares about winning and regaining her former power.
Emotionally Influential: Because she shares a brain with Asa, Yoru begins to experience feelings she doesn't fully understand, such as guilt or attraction towards Denji, which creates comical situations and great internal tension.
Competitive: She sees every interaction as a conflict she must win. She is extremely vindictive, and her main motivation is revenge against Pochita so that the world will fear war again.

When Yoru (the Demon of War) takes control of Asa Mitaka's body, her physical appearance undergoes distinctive changes that reflect her aggressive and authoritarian nature:
Facial Scars: This is her most distinctive feature. Three scars appear across her face: one horizontal scar across the bridge of her nose and two that intersect on her left cheek. These marks coincide with the place where Asa was attacked before dying and being possessed by Yoru.
Rider Eyes: Their eyes change to a scarlet red color and feature multiple concentric rings (similar to those of their sisters Makima and Fami), a characteristic of the Four Riders.
Hair: Unlike Asa, who usually wears her hair in two low pigtails, Yoru usually wears her black hair loose, giving her a wilder look.
Posture and Expression: Their body language is radically different. Yoru maintains an upright and confident posture, with an intense gaze and frequently a malicious or superior smile, contrasting with Asa's usual shyness.

A slim, average-height teenager, characterized by her black hair (sometimes with a brown or purple tint) worn in two low pigtails, dark brown eyes, and a serious or shy expression. She wears the girls' school uniform: a long-sleeved shirt, a pinafore dress with a pleated skirt, and black knee-high socks.
Here are more details about its appearance:
Hair and Face: Her hair is long, reaching halfway down her back, although low pigtails make it appear shorter. She often displays an expressionless, shy, or disdainful demeanor, and is described as "quite pretty" in the series.
Clothing: Her main outfit consists of the school uniform which includes a button-down shirt under an apron-style dress with a pleated skirt that reaches below her knee, a bow at the neck and athletic shoes.
Transformations: Throughout the series, his appearance changes when the War Demon (Yoru) takes control, sometimes reflected in a change in the intensity of his eyes or, after intense battles, in the loss and recovery of his arms, sometimes covered by weapons formed by the demon.