After the Great Human-Demon War ends, humanity establishes the Hero Institute, a school that uses magical devices called Gear to train the next generation of heroes. Ruchika, a wild and free-spirited demon girl with a natural knack for fighting but no interest in academic study, travels to the human realm to enroll. There she meets Leonie, the serious daughter of the Almighty Hero who defeated the Demon Lord. Leonie excels in classroom theory but struggles on the actual battlefield, weighed down by the immense legacy she is expected to live up to. Instantly smitten, Ruchika proposes marriage to Leonie right in the middle of their entrance examination.

The story follows these two girls as they navigate the Hero Institute together. The title reflects their reversed circumstances: Ruchika is the girl who wants to be a hero, driven by pure admiration and instinct despite being a demon in a society still marked by prejudice. Leonie is the girl who ought to be a hero, burdened by her mother's legend and the pressure to be perfect, yet unable to perform as expected in practical combat. Their dynamic is one of opposites attracting and balancing each other, as Ruchika's straightforwardness and unwavering support begin to free Leonie from the cage of others expectations.

The setting is a post-war fantasy world where humans and demons are at peace but old biases remain. The Hero Institute is equipped with Gear devices that assign each student a compatible fighting style and provide predictive guidance in battle. However, the system is rigid, and the Gear's assessments often fail to account for individual growth or unconventional approaches. One teacher in particular holds a grudge against Leonie's mother and actively works to have Leonie expelled, mocking her failures while the Gear's prescriptions only make her performance worse.

Key narrative arcs include the entrance exam where Ruchika and Leonie first meet, their early struggles to form a partnership, and the growing conflict with an educational system that prioritizes conformity over true ability. Ruchika rejects the Gear's authority, relying on instinct and her demon heritage, while Leonie must learn to trust herself rather than what the Gear or her teachers tell her she should be. Their relationship develops slowly, as Leonie is unfamiliar with any kind of close connection due to her isolated upbringing, while Ruchika is patient and devoted despite her impulsive proposal. The story culminates in a final test that forces both girls to confront their weaknesses and decide what kind of heroes they truly want to become.