At the center of Brother Jingi are two young men from opposite ends of the academic and social spectrum. Fujisaki Yuta is a first-year student at Kaisei Gakuen High School, a prestigious national leader in education. Having lost his mother at a young age, he lives a disciplined life with his father and younger sister. In complete contrast stands Eiji, a boy of the same age known as the strongest delinquent at Tsurukame High School, an institution at the very bottom of the academic rankings. Their worlds collide not through choice but by a twist of family fate.

After Yuta helps a girl he likes and gets caught in a fight, he is rescued by Eiji, a rough but capable brawler. This initial, tense encounter seems like a one-time event until Yuta's father drops a bombshell. He announces his intention to remarry a woman named Natsuki, the owner of the snack bar Je t'aime. The catch is that Natsuki is Eiji's mother. Thrown into utter disbelief, Yuta declares his absolute opposition to the union, unwilling to become the brother of someone from a delinquent world he despises.

Despite his protests, the parents' decision forges an unwanted domestic alliance, forcing Yuta and Eiji to live under the same roof. This high-concept setup drives the narrative, exploring a classic odd-couple dynamic in a modern Japanese setting. Yuta is a proud, spineless member of the elite, valuing order and academic achievement, while Eiji operates on a raw code of street justice and loyalty, known as jingi. The central conflict arises from their clashing values and Yuta's refusal to accept this new family.

As the story progresses, several narrative arcs unfold. The initial arc establishes the premise, beginning with their chance encounter and moving through the shock of the remarriage announcement to the forced cohabitation. Subsequent storylines likely delve into the friction at school, where Yuta's reputation could be tarnished by association with a delinquent, and Eiji might be dragged into conflicts involving Kaisei Gakuen's own social hierarchies. The concept also promises a classic arc where the brothers learn to respect each other's strengths, with Eiji's raw power and unexpected code of honor proving useful in situations where Yuta's intellect falters, and Yuta's strategic mind helping Eiji navigate problems that cannot be solved with fists. The core question remains whether these two unwilling siblings can ever truly accept their bond.