Masaru Aoki is a lightweight professional boxer and a core member of the Kamogawa Boxing Gym. He was born on September 9, 1971, in Tokyo, Japan, and works as a chef at a chuka soba restaurant. Aoki stands 171 centimeters tall and fights from an orthodox stance with an unorthodox, creative style.

Aoki's background is rooted in a troubled youth. He spent his childhood and high school years as a delinquent, frequently getting into street fights alongside his longtime friend Tatsuya Kimura. This path changed when he encountered Mamoru Takamura, then a middleweight boxer, and was soundly defeated in a brawl. The loss motivated Aoki to quit school and take up boxing at the Kamogawa gym with the initial goal of landing a clean hit on Takamura. Over time, that rivalry transformed into genuine admiration for Takamura and a deep love for the sport itself.

Aoki's personality is playful, mischievous, and often clownish, making him a primary source of comic relief within the series. He constantly devises tricks and schemes, both inside and outside the ring, and his exaggerated facial expressions add to his humorous presence. Despite this lighthearted exterior, Aoki possesses a fierce loyalty to his friends and gym, along with a stubborn tenacity that surfaces during his most difficult matches. A long-running joke involves his peculiar taste in women, preferring those whom others consider unattractive, a trait possibly stemming from a past rejection by a beautiful girl. He maintains a steady relationship with his girlfriend, Tomiko.

His motivations evolved significantly over the course of the story. What began as a desire to defeat Takamura grew into a genuine pursuit of boxing excellence. Aoki has developed several signature techniques that reflect his improvisational and deceptive approach to fighting. His most famous moves are the Frog Punch, a quirky, leaping punch, and the Look-Away, a trick where he pretends to look elsewhere before landing a surprise blow. These techniques helped him earn a draw in a Japanese title match against the then-champion Imae Katsuka. He also developed a rivalry with Papaya Dachiu, the lightweight champion of the Philippines, fighting him to two consecutive draws.

Aoki's key relationships define much of his character arc. His bond with Tatsuya Kimura is his deepest; the two are frequently referred to as the Aokimura duo and support each other through personal and professional challenges. His relationship with Takamura evolved from initial resentment to mutual respect. Within the gym, Aoki interacts with younger boxers like Makunouchi Ippo as a senior who is both prankish and supportive. He is trained by coach Shinoda.

Aoki's development is marked by moments of intense struggle and renewed determination. A pivotal low point came during his fight against Iga Shinobu, where he was badly beaten and hospitalized. When offered the chance to retire alongside Kimura, Aoki refused, choosing instead to remain in the lightweight class until he could defeat Iga, whom he now considers his primary rival. This decision highlighted his resilience and unwillingness to give up despite a modest record.

In terms of boxing ability, Aoki is not considered exceptionally talented compared to the top fighters in the series. He compensates with creativity, psychological tactics, and an unorthodox style that confuses opponents. He is ranked fifth in the Japanese Boxing Commission lightweight division, with a record of twelve wins, seven by knockout, eight losses, and three draws. While his fights often do not drive the main storyline forward, his personal growth, enduring friendships, and refusal to quit make him a memorable and multidimensional figure in the series.