The central character of this story is Teppei Uesugi, though he is sometimes referred to as Kenichi Uesugi. He is an energetic and rambunctious teenage boy who spent most of his childhood living in the mountains with his father, Hiromi, who was an alcoholic always searching for gold. This isolated upbringing in the wilderness means Teppei has never attended school, cannot read or write his own name, and has absorbed very little of conventional society's rules or manners. He is a wild child, displaying feral behavior that makes him a complete fish out of water when he is forced to move to the city.

Teppei's life undergoes a dramatic shift when his grandmother, with the help of the police, compels him to leave his mountain home and enroll in high school for the first time. He discovers that he actually comes from a prestigious and wealthy family, including a mother named Shinobu and several siblings. Despite his eccentric and often obnoxious behavior, which initially makes him an outsider and a source of constant trouble, Teppei possesses immense natural athletic talent. His physical prowess is especially evident in the sport of kendo, the Japanese art of swordsmanship.

Personality-wise, Teppei is defined by a fearless and reckless nature, a product of his harsh upbringing. He is a pint-sized powerhouse who is made of iron, with endless endurance and the ability to take a tremendous amount of punishment. His attitude is cocky and rude, and he has no problem getting into physical fights, even with his own father, with whom he has a combative but caring relationship. Teppei is an unskilled but strong fighter; he initially ignores the basic rules of kendo but compensates with incredible instincts and raw strength, allowing him to invent his own unorthodox techniques.

Within the story, Teppei's primary motivation shifts from simply surviving his new environment to proving himself. After joining his school's kendo club, his remarkable abilities begin to shine through his rough exterior. He moves from being a disliked loner to a celebrated school hero as others start to admire his tenacity and talent. His key relationships include that with his stern grandmother, who exerts absolute power in the household and is determined to discipline him, yet is one of the first people to truly understand his needs. His relationship with his father is a mirror, as both share the same brash, ignorant, and violent tendencies. He also forms a friendship with a roommate named Gonda, a more sensitive student who is initially bewildered by Teppei but comes to understand him. His primary guide in the competitive world of kendo is the club's captain, Yoshioka, as well as his teacher and advisor, Professor Ishiwata.