Miyu Mido is the older sister of protagonist Miko Mido and works as a drama actress. Demons from the Shikima realm abducted Miyu, triggering her rescue by Miko. It was later revealed that Miyu and Miko share the same mother, Maria, but have different fathers. Miyu was born from Maria's union with a member of the rival Suzuka clan, while Miko was fathered by Seikima, the King of the Shikima demons. This lineage difference carried consequences: Maria's marriage to a Suzuka clansman disqualified her from being chief of the Miroku ninja clan, leading to her exile. Following this, Miko was entrusted to her grandmother for ninja upbringing and training. Miyu's circumstances during this period remain unspecified.

In later installments, specifically during *La Blue Girl Returns*, Miyu became romantically involved with a man named Rokuga. Unbeknownst to Miyu, Rokuga belonged to the Mahoroba clan, an insectoid ninja group with hostile intentions. His relationship with Miyu was a ruse to gain access to the Shikima seal case, which he successfully stole. This theft was part of the Mahoroba clan's larger scheme to conquer the Shikima Brain, a legendary location within the Shikima realm rumored to grant dominion over the universe. Miyu's role underscores her continued vulnerability to manipulation by adversaries exploiting her connection to the Miroku clan and Shikima artifacts.