Mizuki Shido is the devoted older sister of Masaki Shido, a skilled Gundam Battle Nexus Online player. After Masaki's consciousness became trapped within the game during a mission on the planet Eldora, plunging him into a coma, Mizuki assumed responsibility for his care. She regularly visits her unconscious younger brother at the hospital.

During one such visit, she encounters Hiroto Kuga, Kazami Torimachi, May, and Patrick Alexandre Leonard Arge, who identify themselves as Masaki's friends from Gundam Battle Nexus Online. Mizuki shares insights into Masaki's real-life character, depicting him as a Gunpla enthusiast who typically acted independently yet possessed a fundamentally caring nature, always willing to help others in need. She recounts that before his coma, Masaki mentioned participating in something significant within the game, specifically his involvement with the Eldoran Resistance. He expressed confidence in protecting others by capturing a critical objective. To corroborate his activities, Mizuki shows the group a photograph from Masaki's phone, depicting him on Eldora alongside Resistance members.

Her role remains centered on supporting her comatose brother.