Mikuru Shindou is Rive Shindou's sister, featured in the Shinemon Arc set ten years before the main story. As a middle school student at Tokyo Metropolitan Harumi, she has long orange hair, blue eyes, and wears the school uniform: a pink jacket, white skirt, and boots.

Cautious and quick to anger, Mikuru reacts strongly to reckless behavior, particularly targeting Shinemon Nitta whom she deems irresponsible. Despite her frustrations, she shows protective instincts towards her brother Rive and others connected to Card Capital. Her role centers on Card Capital's early struggles. When Esuka Hibino threatens a takeover, Shinemon positions himself as the shop's protector. In one incident where Shinemon leaves the shop to his disciple Tatsuya Tachibana, antagonists Nanami Gonomi and Tonori Fujinami attempt to forcibly remove Mikuru, highlighting her vulnerability and the threats facing the Card Capital community.

A subplot hints at a potential idol career for Mikuru. Her familial bond with Rive later gains significance in the broader series timeline as Rive is Chrono Shindou's father. Mikuru's cautious nature and protective disposition persist throughout her appearances, shaping her interactions with Shinemon and her brother.