Ipponmatsu is a supporting character who appears in the Loguetown Arc of the story. He is the owner of a long-established weapons shop in Loguetown, a business that has been in his family for two hundred years. As an older man with a red nose and a unique hairstyle consisting of long sideburns and a small crest of hair on his forehead, he is a resident of the East Blue. His primary occupation is running his shop, though he laments that business has suffered greatly ever since the Marine captain Smoker was assigned to the town, as the decrease in pirate activity has led to fewer customers.

Ipponmatsu is a complex individual who initially comes across as greedy and opportunistic. When Roronoa Zoro visits his shop looking for two swords with only one hundred thousand berries to spend, Ipponmatsu is dismissive, seeing Zoro as a naive customer. Upon recognizing the great value of Zoro's sword, the Wado Ichimonji, he attempts to trick the swordsman into selling it for far less than it is worth, a scheme that is foiled by the arrival of the Marine swordswoman Tashigi. However, this greedy exterior conceals a deep knowledge and genuine love for swords. He has extensive knowledge of famous blades and their histories. His attitude changes completely when Zoro demonstrates his courage and will by testing the cursed blade Sandai Kitetsu. Zoro tosses the sword into the air and holds out his arm, risking amputation to prove his luck and mastery over the weapon. Impressed by this display of a true swordsman's spirit, Ipponmatsu apologizes for his earlier behavior and makes a remarkable decision.

Moved by Zoro's conviction, Ipponmatsu gives him the Sandai Kitetsu for free. Not stopping there, he also presents Zoro with Yubashiri, a treasured family heirloom and one of the finest swords in his shop. His motivation is not financial gain but the passing of a dream. He states that a man entrusting his dream to another man is a noble act, revealing a core part of his personality that values honor and the spirit of a swordsman above profit. This action shows he is a fan of "bad guys" or, more accurately, of strong-willed individuals who live by their own code. Despite his earlier shrewdness, he is ultimately a sentimental man who respects genuine skill and bravery.

Ipponmatsu has a key relationship with his wife, whom he affectionately calls "honey" or "dear". Despite his sometimes stingy nature, he is a devoted husband who is willing to do chores she asks of him, such as cleaning the bathroom. His admiration for Zoro continues long after the swordsman leaves Loguetown. Years later, he is seen cleaning his bathroom while proudly displaying a large poster of Zoro's wanted picture on the wall, showing that he still feels a strong connection to the man to whom he entrusted his dream. He has a less favorable view of the Marines, particularly Smoker, whom he considers a monster for driving away his customers, and Tashigi, whom he views as a clumsy hazard.

Ipponmatsu does not possess any supernatural abilities or fighting skills of his own. His notable abilities are entirely practical and professional. He has a sharp eye for the quality and history of swords, being able to identify a rare blade like the Wado Ichimonji on sight. He also has the knowledge and skill to properly maintain and sharpen swords, as he regularly does for Tashigi's weapon, Shigure. His role in the story is that of a mentor figure and a benefactor, providing Zoro with the tools he needs for his journey into the Grand Line and acting as an example of an East Blue civilian who supports the next generation of pirates.