Mejiro Ryan is a sporty and energetic member of the prestigious Mejiro family who has a deep and abiding love for physical training and weightlifting. Born on April 11, she is a student at Tracen Academy with a distinctive appearance marked by very short, spiky brown hair and a prominent white tuft, a style known as the "Ryan Cut" inspired by the real-life racehorse's closely clipped mane. Her usual attire includes a practical white and teal striped racing outfit that has elements of a baseball uniform, a reference to the horse's namesake, the Major League Baseball player Nolan Ryan. Despite her lean and athletic build, she notes her weight has seen a slight increase due to the effectiveness of her consistent workouts.

While her energetic and cheerful demeanor makes her popular at school, her personality is more complex than it first appears. Although born into the illustrious Mejiro family, known for its grace and success, Ryan's animated and straightforward manner somewhat betrays that refined image. She is passionate about building muscle and frequently encourages others to join her in getting "swole," believing that most problems can be solved through physical strength. However, beneath this tough, fitness-oriented exterior lies a surprisingly delicate and sensitive heart. She harbors a secret fondness for feminine and cute things, such as reading shoujo romance manga, though she often feels shy about these interests and worries they do not suit her image. Her ears are known to move around a lot when she is engrossed in her favorite romance stories.

A central and driving aspect of Ryan's character is her relationship with her childhood friend, Mejiro McQueen. She holds a deep admiration for McQueen, viewing her as the embodiment of the Mejiro family's ideals of speed and elegance, qualities she herself strives for. This admiration, however, is intertwined with a powerful inferiority complex, as Ryan often feels she cannot compare to McQueen's natural talent and grace. This perceived gap serves as her primary motivation, pushing her to dedicate herself to racing in an effort to overcome her rival and prove her own worth. Her key relationships are largely centered within the Mejiro family circle, including connections to Mejiro Dober and Mejiro Bright. At the academy, her roommate is Ines Fujin, with whom she shares a friendly rivalry. She also holds a tremendous amount of respect for Oguri Cap, looking up to her as a source of inspiration.

Within the story, Ryan is defined by her competitive rivalry with McQueen and her persistent chase for a prestigious G1 victory. She is portrayed as a talented runner who often found herself finishing just behind her more celebrated family member, a dynamic that has a notable impact on her confidence. A significant moment in her development occurs during the 1991 Takarazuka Kinen, where she finally manages to secure a victory against Mejiro McQueen, achieving a long-held goal. Her personal journey is one of reconciling her own unrefined, muscle-focused identity with her desire to be as fast and pretty as the ideal she sees in McQueen, learning to accept her unique strengths in the process.

In terms of racing abilities, Ryan excels as a runner who can employ a late-race surge or pace-making strategy, performing best on turf and over medium distances. Her physical strength is her greatest asset, and this is reflected in her unique skill, "Let's Pump Some Iron," which significantly increases her acceleration when she attempts to move up through the pack on a corner during the latter stages of a race. This skill underscores her ability to use her raw power for a decisive, game-changing burst of speed when it matters most.