Ippo Makunouchi is introduced as a sixteen-year-old high school student living a constrained life. Having lost his father at a young age, he dedicates most of his time to helping his mother run their family's fishing boat rental business. This responsibility leaves him feeling distant from his peers and without a clear direction in life, causing him to endure relentless bullying from classmates who take advantage of his quiet and non-confrontational nature.

His personality outside of the ring is defined by a profound shyness and humility. Even after becoming a successful boxer, he remains a kind, reserved, and awkward young man who dislikes unnecessary fighting and often humbles himself around his rivals. He continues to be intimidated by former bullies and fears certain opponents he has already defeated, such as Ryo Mashiba. This gentle demeanor extends to his personal life, where he is a late bloomer in romance, harboring pure-hearted and awkward feelings for Kumi Mashiba while struggling to progress in their relationship. Despite his size and muscular frame, he is the type of person who would rather use his newfound stamina to run away from punks than to confront them violently outside of the ring.

However, this meek exterior completely transforms once Ippo steps into the ring. There, he becomes an intense and fiercely determined fighter with remarkable courage and a willingness to push himself to his absolute maximum. His motivation for boxing stems from a deep, philosophical desire to understand what it means to be strong, a question that drives him to pursue a professional boxing career under the guidance of the Kamogawa Gym. Another powerful motivation is his desire to one day face his eternal rival, Ichiro Miyata, in a professional match, a goal that shapes many of his decisions throughout his career.

In the narrative, Ippo serves as the central protagonist whose journey from a bullied teenager to a champion boxer is the core focus. His role is defined by his rapid growth and his defenses of the Japanese Featherweight Championship. Key relationships heavily influence his development. His bond with his coach, Genji Kamogawa, is a deep and trusting teacher-student relationship where Kamogawa becomes a surrogate father figure, instilling in Ippo the discipline and techniques that define his style. He shares a brotherly bond with his gym mate, Mamoru Takamura, who rescued him from bullies and introduced him to the world of boxing. The relationship with Miyata is that of mutual respect and rivalry, with Ippo viewing him as a friend and an ultimate goal.

Ippo's development is marked by his mental growth as much as his physical prowess. While he begins with no training, his years of physical labor have gifted him with a sturdy body, powerful legs, and a naturally powerful punch, often called a "dynamite punch". His coach hones this natural talent, teaching him to fight as an in-fighter who excels at close-quarters combat. His notable abilities include a devastating Liver Blow, a leaping uppercut known as the Gazelle Punch, and the signature Dempsey Roll, a technique where he weaves in a figure-eight pattern to unleash a torrent of powerful hooks. Beyond these physical tools, Ippo's greatest assets are his incredible stamina and his capacity to endure any amount of punishment, getting up from knockdowns repeatedly to continue fighting. Over time, he also develops a keen sense of observation and insight, able to notice details about opponents even outside the ring.