Mio Mibuya is a character from Gunparade March, set in an alternate 1999 where humanity has been fighting alien creatures known as Genjuu for over fifty years. She is a sixteen-year-old high school student who serves as a pilot of a Humanoid Walking Tank (HWT) in the Japanese military. Standing 1.62 meters tall and weighing 42 kilograms, she has long black hair and blue eyes. Her role in the story is that of a supporting pilot within the unit, where she is known for her disciplined and diligent nature. She frequently uses the phrase "That's filthy!" as a personal catchphrase, reflecting her fastidious personality.

Mio pilots an HWT that is uniquely equipped with a sword, unlike the standard armaments of her teammates. This sword is used in combat to cut an opening into the brain of a Genjuu, allowing a PBE bomb to be inserted for a kill. While she is a capable pilot, she is not especially proficient with ranged weapons and prefers to charge into battle with her sword in both hands. This aggressive fighting style is complemented by her preference for heavy armor on her HWT. However, her attachment to her sword is portrayed as a sign of mental immaturity—the weapon was originally a spirit sword, but its effectiveness is hampered by her own psychological limitations. She is also shown to be easily influenced by political maneuvering, which affects her judgment and actions.

Mio's primary interpersonal dynamic is a rivalry with Mai Shibamura, a fellow pilot who joins the unit later. This rivalry drives both characters to push themselves in training and combat. Despite her stern exterior, Mio is deeply committed to her duty and the defense of humanity. Her story arc culminates tragically when her HWT is damaged in battle by a Genjuu, exposing her to contact with the alien creature. She becomes poisoned from this contact and dies soon afterward, marking a significant loss for the platoon. Her death underscores the harsh realities of the ongoing war and the sacrifices made by the young soldiers.