Mari Mizuno is a supporting character in Masamune-kun's Revenge R. She is one of Aki Adagaki's self-proclaimed bodyguards and the primary strategist of a small group that includes her two companions, Kikune Kiba and Kaneko Sonoka. She serves as the class representative for Class 2-A at Yasaka Private High School and is also a classmate of Aki. Mari is easily recognized by her distinctive white hairband and red full-frame glasses.

Her personality is calm, composed, and highly analytical. She approaches situations with a strategic mindset and is the brains behind most of the schemes directed at Masamune Makabe. Mari holds a strong sense of loyalty toward Aki and is deeply protective of her, which fuels her distrust and dislike of Masamune. She believes that Masamune is not a suitable companion for Aki and actively works to keep him away from her. At one point in the story, she is deceived by Kanetsugu Gasō's false claims and attempts to obstruct Masamune's efforts, though her schemes are ultimately part of the larger comedic and romantic conflicts.

Mari's role in the narrative is that of a minor antagonist and obstacle for the protagonist. She operates primarily from the background, coordinating plans and leveraging her position as class representative to monitor or interfere with Masamune's activities. Her motivations are rooted in protecting Aki's interests and maintaining the status quo, as she views Masamune as a threat to Aki's wellbeing. Over the course of the series, Mari shows little personal growth or change, remaining steadfast in her role as a loyal guardian and schemer. Her notable ability is her tactical acumen, which allows her to devise and execute plots, though these often fail due to the unpredictable nature of the main characters.