Bon Namihira is the main protagonist of the story, a Japanese junior high school student whose name deliberately signifies "mediocre," "ordinary," or "average." This naming is a key aspect of his identity, as he openly admits to having no exceptional academic, athletic, or social skills and claims to actually like being average at everything. He is neither particularly lonely nor sociable and has no strong interests to devote himself to, though he does possess a strong sense of curiosity. Physically, he is an average teenager, initially depicted as slightly shorter than his colleague Ream Stream before later matching her height, and his design varies across different adaptations of the story.

Bon's background is that of an unassuming student whose life changes dramatically after an accidental discovery. While relaxing at his friend Tetsuo's house, Bon witnesses Tetsuo fall from a deadly height and die. Before he can process this, reality warps and he finds himself sitting next to a living Tetsuo as if nothing had happened. This leads him to meet Ream Stream and her partner Buyoyon, who explain that he has witnessed time travel. The Time Patrol agents conclude that Bon must be erased from existence to keep their secret safe, but they discover that he is "historically relevant" and cannot be removed without risking a major change to the course of history. The reason for his relevance is not a grand, heroic act but a seemingly thoughtless action: he once kicked a rock that led to a chain of events saving a future politician who would avert a third World War. With no other option, Bon is forced to abandon his average existence and join the Time Patrol as a trainee agent, working under the instruction of Ream.

In terms of personality, Bon is portrayed as clumsy, careless, and occasionally cocky, which often leads to him making pathetic mistakes and causing self-inflicted injuries, even after becoming an agent. He is somewhat lazy, though generally more diligent than similar characters. However, despite his average nature, Bon exhibits remarkable bravery in emergencies, such as when he faces his own near-execution by the Time Patrol or rescues Ream from a prehistoric creature. This courage can also lead to reckless actions, resulting in his death on more than one occasion; for instance, he was killed by the Minotaur and artificially died from cardiac arrest to replace the historical figure Philippides, thereby becoming the new sire of the marathon under the name Eukles. His strong moral compass and sense of justice frequently clash with the strict regulations of the Time Patrol, which forbid interfering with the deaths of historically significant people, causing him significant emotional conflict.

Bon's role in the story follows his development from a novice prone to errors to a more capable agent. His abilities, true to his name, are generally unexceptional. He has poor grades and middling stamina, and he is neither a star player nor a burden on his school's baseball team. However, he possesses a few hidden talents. Most notably, he has an unusual and exceptional talent for marksmanship, which he uses to save a character from committing murder. He also displays high adaptability in crises and surprising agility and endurance when his life is on the line, managing to survive ambushes and complete the original marathon from Marathon to Athens under extreme conditions. Unlike his partner Ream, Bon is also a capable swimmer.

His key relationships drive much of his development. His dynamic with his instructor, Ream Stream, is complex and carries ambiguous romantic undertones that were left unresolved due to the manga's cancellation. Their partnership eventually comes to an end when Ream transfers to another unit and Bon is promoted to a full agent. Following this, he is paired with Yumiko Yasukawa, a girl he originally saved on a mission who later joins the Time Patrol, with Bon acting as her senior agent. He maintains friendships with his schoolmates Tetsuo Shiraishi and Yanagisawa, though Tetsuo often unintentionally mocks him, and he keeps his time-traveling adventures a secret from them, making his life seem average to outsiders. Throughout his missions, he transitions from a reckless novice who breaks rules to an agent who understands their importance, all while remaining committed to preserving life within the limits permitted by the Time Patrol. His actions inadvertently influence history, such as inspiring the legend of the Monkey King, and he experiences altered memories due to the time paradoxes he creates.