Junko Hattori is a central character in the story, serving as a student at Constant Magical Academy and the class representative for class 1-A. She comes from a prestigious and long-standing family, the Hattori Clan, which functions as a samurai-like group of ninja warriors charged with protecting the Empire for generations. As a member of this clan, she is also a devoted disciple of the god Suhara, the deity her family serves.

Personality wise, Junko is defined by a powerful and unwavering belief in justice. She is very dedicated to her family's traditions and her role as a guardian of order. This serious and upright nature can make her appear harsh or abrasive at first, but she is capable of warming up to people over time. A key aspect of her personality is a tendency to misinterpret the actions and good intentions of others, particularly the protagonist Akuto Sai, frequently seeing them as evil or perverse when they are not. This trait, combined with her passionate sense of justice, makes her a tsundere character—initially hostile and cold towards Akuto before developing deep romantic feelings for him later in the story.

Junko's primary motivation is rooted in her duty to the Hattori Clan and her faith in the god Suhara. She initially meets Akuto Sai on the train to school, where the two form a pact to make the world a better place. However, when a magical placement test declares that Akuto is destined to become the next Demon King, she feels profoundly betrayed, believing he tricked her. Her motivation shifts from being his ally to being his enemy, as she decides it is her duty to destroy him for the good of society. Over the course of the series, as she witnesses his genuine attempts to do good, she begins to develop strong feelings for him, becoming conflicted between her duty to destroy the Demon King and her personal affection for Akuto.

In the story's narrative, Junko acts as a secondary protagonist and a major foil to Akuto. She is often the one who most publicly opposes him and challenges him to duels, creating much of the initial conflict. Her role is also that of a close friend and eventual love interest, forming a key part of the romantic subplot. Several key relationships define her role. Her relationship with Akuto Sai is the most important; it evolves from a brief alliance to fierce antagonism, and finally to a deep romantic bond. She was also assigned to monitor him alongside Korone, another observer, creating a complex dynamic. She has a younger sister named Yuuko Hattori, a pop star idol, and a father named Youzou Hattori, the head of the clan from whom she receives her primary weapon.

Over the course of the series, Junko undergoes significant development. She begins as a rigid and duty-bound individual who sees the world in black and white, but her experiences with Akuto force her to confront the nuances of justice and destiny. Her initial hatred for the proclaimed Demon King slowly gives way to understanding and love as she realizes his true character. She must reconcile her lifelong faith in the god Suhara and her family's mission with her own personal feelings, ultimately choosing to fight for the man she loves.

In terms of abilities, Junko is a highly skilled and powerful fighter, ranked second in the school's fighting rankings. As a member of a legendary ninja clan, she is a master of ninjutsu and Hattori-style swordsmanship. Her signature technique is a clone technique that allows her to create multiple copies of herself to attack an opponent. Her primary weapon is a treasured sword named Sohaya no Tsurugi, a sacred blade passed down through the Hattori family that is associated with the god Suhara. The sword has the ability to amplify the user's internal mana several times over. She can summon this sword at any time. In addition to her sword, she wields kodachi (short swords) and throwing weapons like shuriken and kunai.