Kinro is a young man who serves as one of the guards of Ishigami Village alongside his younger brother, Ginro. He is a descendant of the astronauts who survived the global petrification event 3,700 years prior to the story's main events. At the time of his introduction, he is eighteen years old. Physically, he has a muscular and athletic build, with brown hair and green eyes. A distinctive scar marks his right cheek, and a long lock of hair often hangs between his eyes. His typical attire consists of a sleeveless uniform resembling a karate gi, secured with a rope belt, and he carries a buckler on his back that is emblazoned with the village's star pattern.

In terms of personality, Kinro is defined by his stern, serious, and uncompromising nature. He takes his responsibilities as a gatekeeper with the utmost gravity and is a firm believer in hard work, discipline, and adherence to the rules. He holds a deep disdain for laziness, cheating, or any form of dishonesty, regardless of how noble the cause might be. This unwavering commitment to order initially makes him suspicious of Senku Ishigami and the unfamiliar "sorcery" of science. He is also incredibly proud, to the point of a fault. He secretly suffers from severe nearsightedness, which villagers call the "blurry eye disease," but he refuses to disclose this weakness because he believes a man should offer no excuses for his failures, even though it puts him at a significant disadvantage in combat. Despite this gruff and rigid exterior, Kinro is capable of deep emotional attachments and can be swayed by sentimentality. He shows a strong, almost childlike fondness for his gilded spear, which Senku gave him as a bribe, and he is genuinely excited by new scientific inventions like an automated generator or an automobile.

Kinro’s primary motivation is the faithful execution of his duty to protect Ishigami Village and uphold its laws. His actions are driven by a sense of honor and loyalty to his home and its people. While he initially sees Senku as a threat, his perspective shifts as he observes the scientist's resolve and his efforts to help the village. He becomes motivated by a desire to see good triumph, even if it means cooperating with methods he does not fully understand. His role in the story evolves from that of a minor obstacle and local authority figure to a key combatant within the Kingdom of Science. He becomes a central member of the "Power Team," lending his significant physical strength to Senku's endeavors. Throughout the series, he participates in major conflicts, including the village tournament to decide the next chief, the battles against the Tsukasa Empire, and the expedition to Treasure Island where he is temporarily turned to stone.

The most significant relationship in Kinro's life is with his younger brother, Ginro. The two are polar opposites in almost every way: Kinro is diligent, responsible, and serious, while Ginro is lazy, cowardly, and self-interested. Despite this, they share an incredibly deep and unbreakable bond. Kinro constantly lectures Ginro on his shortcomings, often expressing frustration with what he perceives as a lack of discipline. However, this stems from a place of caring, as he genuinely wants his brother to improve. This loyalty is mutual; when Kinro is gravely injured during an attack and offers to sacrifice himself by having the bridge he is on cut down, Ginro refuses and breaks down crying, unable to abandon him. Kinro, in turn, expresses pride when he learns of Ginro's own acts of bravery. His relationship with Senku also develops significantly. After initially refusing him entry to the village, Kinro eventually becomes one of Senku's staunchest defenders, vouching for his character to the other villagers and putting his trust in the scientific method, particularly after receiving a pair of glasses that correct his eyesight.

Throughout the series, Kinro undergoes notable development, primarily in softening his rigid worldview. A key turning point is the village tournament, where he accepts and uses a helmet lens provided by Suika to improve his vision during a fight. Although he is disqualified due to a technicality, this experience opens his mind to the practical benefits of science. He overcomes his stubborn pride by accepting a pair of glasses from Senku, a change that not only improves his combat effectiveness but also symbolizes his acceptance of a new way of thinking. He learns that innovation is not a form of cheating but a tool that can enhance one's natural abilities and aid the community. By the story's end, his sense of duty remains intact, but it is now coupled with a more flexible and modern understanding of the world.

Kinro’s notable abilities lie in his physical prowess. He is acknowledged as one of the strongest and most capable fighters in Ishigami Village, with his power rivaled only by elite warriors like Kohaku and Magma. He is a highly skilled spearman, wielding his weapon in combat with great technique and effectiveness. After his nearsightedness is corrected with glasses, he is able to fully utilize his abilities, becoming an even more formidable opponent. In addition to his spear, he uses a buckler for defense. His strength and endurance are considerable, allowing him to battle against powerful enemies from the Tsukasa Empire. His unwavering dedication to his duty also serves as a form of mental fortitude, driving him to continue fighting even when severely injured.