Eita Kidō is the protagonist of his story, a first-year high school student known for his sharp mind and his deep-seated hatred of romance. He is a diligent and academically gifted young man who consistently ranks at the top of his class. This intense focus on his studies is driven by a singular, altruistic goal: to gain admission to a medical school and eventually become a doctor. The primary motivation for this career path is his childhood friend, Chiwa Harusaki, whose injury prevents her from continuing the kendo she once loved. Eita studies relentlessly in the hope of one day finding a way to cure her.

Eita's rejection of romantic love is not a simple matter of teenage angst but a direct result of his painful family history. His parents, whom he describes as acting like "love-struck patients," went through a bitter divorce and subsequently abandoned him to pursue new relationships. This traumatic event, which occurred during his third year of middle school, left him feeling alone and responsible for himself, and it solidified his belief that love is unnecessary and dangerous. He proudly identifies as "anti-love" and is determined to avoid any situation that might lead to a romantic entanglement.

Despite his current studious and serious demeanor, Eita has a colorful and embarrassing past. During his middle school years, he suffered from a severe case of "chūnibyō," a form of delusional behavior where he proclaimed himself to be the "Burning Fighting Fighter" and filled a notebook with grandiose fantasies about past lives and special powers. He has since tried to bury this part of his life, but his past comes back to haunt him when the beautiful and equally cynical transfer student, Masuzu Natsukawa, discovers the notebook. She uses it as blackmail material to force him into a fake relationship, threatening to expose his humiliating secrets to the whole school if he refuses.

His role in the story is largely defined by this forced arrangement. As Masuzu's fake boyfriend, he becomes the unwilling center of attention, finding himself in a chaotic "battlefield" between his fake girlfriend, his concerned childhood friend, and other girls from his past who re-enter his life. His relationships are complex and central to the plot. With Masuzu, he begins a reluctant alliance built on blackmail, but over time, he grows to truly care for her and understand the personal pain that fuels her own cynical worldview, eventually wanting to stay by her side even if their relationship is a sham. His relationship with Chiwa is one of deep, familial care, as he is her primary emotional support and has dedicated his career to her well-being, though he remains oblivious to her romantic feelings for him. Two other key relationships emerge: Ai Fuyuumi, another childhood friend who claims he made a marriage promise to her in kindergarten, and Himeka Akishino, a shy transfer student who believes she and Eita were lovers in a past life after witnessing him re-enact one of his old chūnibyō delusions.

Throughout the series, Eita shows signs of development, particularly in his rigid stance on love. While he continues to assert that he has no romantic feelings for anyone, his actions reveal a growing emotional investment, especially toward Masuzu. He moves from a position of pure coercion to one of genuine care and protectiveness. Aside from his academic prowess, Eita possesses the unexpected ability to tap into his old chūnibyō persona when the situation calls for it, using his over-the-top antics to save the day or help a friend in distress. He also manages his aunt Saeko's household largely on his own, as she is a game creator who is rarely home, which has made him highly skilled at general housework.