MIO5, whose full name is Koumori Mioko, is a supporting character in the anime series Ojisan to Marshmallow. She is a thirty-year-old illustrator working at the same web-related company as the characters Hige-san and Wakabayashi. Her nickname MIO5 is a play on her given name Mioko, as the character for five in Japanese can be read as ko. She is also sometimes called Degasu-chan because she habitually ends her sentences with the regional expression degasu, reflecting her distinct regional accent.

Her motivations are largely tied to her personal relationships. She developed a romantic interest in Hige after he inadvertently saved her from a groper, an event that sparked her feelings for him. Additionally, she is revealed to be the girlfriend of the company’s president, which adds another layer to her connections within the workplace.

In the story, MIO5 serves as a colleague and occasional love-interest figure, contributing to the office dynamics and romantic entanglements that drive the series' comedic and heartfelt moments. Her background as an illustrator informs her role, though her notable abilities are not extensively elaborated beyond her professional skills. Throughout the series, she is portrayed as a warm, slightly quirky character whose regional speech and affectionate nature make her a memorable part of the ensemble.