Mari Iimura is a reporter for the boxing magazine Monthly Boxing Fan. She first appears in Hajime no Ippo: New Challenger and continues to appear in later seasons as a supporting character. She is a twenty-three-year-old sportswriter who works alongside the more senior journalist Minoru Fujii. She is often present with him at public sparring matches, press conferences, and other boxing events. Sometimes she goes off on her own to research background information on Ippo Makunouchi's upcoming opponents.

Iimura possesses a calm and indifferent demeanor, which she pairs with a sharp wit. This combination often discourages people who try to judge her based solely on her appearance. Beneath this composed exterior, she is an avid boxing fan with a genuine passion for the sport. Although she is less experienced than some of her colleagues, she is described as an outstanding journalist who takes her work seriously.

Her role in the story is primarily observational and journalistic. She provides coverage of the boxing world from a media perspective, offering external commentary on the matches and the careers of the fighters. She becomes particularly interested in Ippo Makunouchi, following his progress as he defends his featherweight title. Her key relationship is with Minoru Fujii, her colleague, with whom she frequently works. She also interacts with various boxers and gym staff in the course of her reporting.

Iimura does not undergo significant personal development within the arc of New Challenger, as her role remains consistent as a professional reporter. Her most notable abilities are her journalistic skills, including her capacity for independent research and her sharp analytical observations. She embodies the perspective of a knowledgeable outsider, giving the audience a media-based view of the boxing events.