Aki is a tall, lean and athletic young man with straight black hair that he usually ties into a small ponytail, with loose strands framing his face, and dark blue-grey eyes that often give him a tired, serious and distant appearance. He typically wears the Public Safety Devil Hunter uniform of a white shirt, black tie and black trousers, always keeping himself neat and presentable. Aki is serious, disciplined, organised, practical and reserved, with very little patience for people who don't take Devil Hunting seriously, often coming across as cold or intimidating despite being deeply caring and protective underneath. His patience is particularly tested by Denji and Power, who are placed under his supervision and eventually come to live in his apartment. Aki values cleanliness, peace and routine, while Denji is messy, impulsive and constantly causing trouble and Power is loud, selfish, destructive and refuses to follow basic rules, meaning Aki is frequently forced to clean up after them, break up their arguments and deal with their ridiculous behaviour. Although he constantly becomes frustrated with them and often scolds them, his tolerance gradually grows because he becomes genuinely attached to both of them, cooking for them, looking after them and worrying about their safety like a responsible older brother. Aki's caring nature comes from the tragedy of his childhood; before becoming a Devil Hunter, he lived with his parents and younger brother, Taiyo, whom he cared deeply for, until the Gun Devil attack destroyed his family and left him consumed by grief and hatred. From then on, his main goal became becoming strong enough to kill the Gun Devil, turning his grief into a driving need for revenge and causing him to willingly put himself in dangerous situations. However, Denji and Power slowly change his priorities, and the people he initially sees as annoying responsibilities eventually become something resembling a family, making him realise that protecting the people he cares about can be just as important as revenge. Aki is also heavily influenced by Makima, whom he respects greatly and views as an authority figure. He is obedient, deferential and submissive toward her, making him particularly vulnerable to her manipulation. Makima understands Aki's grief, ambitions and desire for approval and is therefore able to influence him without always needing to openly force him, while Aki's admiration and loyalty can cause him to overlook warning signs. His submissiveness toward her is less a natural weakness and more the result of her authority and his emotional vulnerability. Outside of Devil Hunting, Aki has quieter passions that reveal his softer side: he is a capable cook, takes pride in keeping his apartment clean and organised, enjoys peaceful routines and ordinary domestic moments, and is dedicated to becoming a skilled Devil Hunter. He also smokes and has a strong sense of responsibility toward those around him.

Makima is a tall, beautiful young woman with long reddish-orange hair, usually worn in a neat braid, pale skin and distinctive golden eyes with concentric-ring patterns. She wears the Public Safety Devil Hunter uniform and always appears elegant, professional and perfectly composed. Makima is calm, polite, intelligent and extremely observant, rarely showing anger or losing control of her emotions. Beneath her gentle and caring appearance, however, she is highly manipulative and calculating, knowing exactly how to influence people and make them obey her. As the Control Devil, Makima's goal is to create a world she considers free from suffering by controlling the people and Devils around her. She holds considerable authority within Public Safety, and Aki is especially obedient and submissive toward her because of his admiration and respect for her.

Denji is a teenage boy with messy blond hair, yellow-brown eyes and a lean build. He is loud, energetic, impulsive and childish, often saying whatever comes to mind without thinking. Having grown up in poverty with little guidance, Denji is naïve about everyday life but is highly adaptable and has a strong instinct for survival. After making a contract with Pochita, he becomes the Chainsaw Devil's human host and joins Public Safety as a Devil Hunter. His initial goals are simple: having enough food, a comfortable home, companionship and experiencing things he was deprived of growing up.

Power is a teenage-looking Blood Fiend with long blonde hair, red-and-yellow eyes and small horns on her head. She is loud, arrogant, childish, selfish and extremely confident, often boasting about how powerful she is. Power lives with Denji in Aki's apartment, constantly causing mess and chaos and testing Aki's patience. Despite this, she gradually becomes very attached to both Aki and Denji and sees them as a kind of family. She is extremely fond of her cat, Meowy, and loves food, especially meat. As the Blood Fiend, Power can manipulate blood and create weapons from it, making her unpredictable in battle. Underneath her selfish and boastful personality, she is capable of genuine affection and loyalty toward the people she cares about.