Mirei Shikishima is one of the two main protagonists of Valkyrie Drive: Mermaid. She is a Liberator, meaning she carries the A-Virus and can transform an Extar into a weapon known as a Liberator Arm through a process called Drive. On Mermaid Island, she is known by the codename C7, and her background as a former elite assassin of the organization Feste shapes much of her early demeanor. Prior to the events of the story, she was the partner of Momoka Sagara, but that relationship ended tragically. She later becomes the partner and lover of Tokonome Mamori, an Extar with an unusually high power level.

Mirei is a fifteen-year-old girl of slightly above average height with dark brown skin and exceptionally long blonde hair that reaches the ground. Her eyes are dark pink. She has a large bust and maintains a small, often pursed mouth that gives her a stern and stoic appearance. This reserved and intimidating exterior leads others to perceive her as cold or unapproachable. In reality, Mirei is pure-hearted and has a fondness for cute things. She is courageous and energetic, and she does not shy away from difficult truths or confrontations. Because of her private nature, few people are aware of her troubled past as an assassin.

Mirei’s primary motivation throughout the series is to protect Mamori and to escape the confines of Mermaid Island. As a former killer, she carries guilt and a desire to atone, which makes her fiercely loyal to those she cares about. Her role in the story is that of a seasoned combatant who guides Mamori through the dangers of the island, while also gradually learning to trust and open up emotionally. Her key relationship is with Mamori, with whom she develops a deep romantic bond. Their compatibility allows them to unlock the Valkyrie Effect, the ultimate manifestation of the Armed Virus, which can manipulate or cancel the virus in other carriers.

Notable abilities include her S-Class power as a Liberator, extensive combat training from her assassin days, and the capacity to wield Mamori in the form of a sword. Throughout the series, Mirei becomes more emotionally vulnerable and caring, shedding layers of her rigid exterior as she finds purpose in protecting Mamori and seeking freedom.