Yaku is a surviving kunoichi of the Miroku ninja clan, escaping the clan's destruction alongside Miko Mido after the massacre orchestrated by traitors Kamiri and Kugutsumen. She possesses a unique curse: failure to achieve sexual satisfaction before a full moon triggers a dangerous transformation into a werewolf. Physically, Yaku has striking green hair reaching her shoulders, red eyes, and often wears a hair ribbon. Her mature demeanor contrasts with Miko, making her seem older despite their similar age, and her large breasts are a noted point of envy for Miko. Sexually active, Yaku favors partners with a delinquent appearance, though they frequently struggle to match her intense carnal energy. She displays confidence in combat and a strong protective instinct, especially towards Miko. In conflicts against threats like the Shikima demons, rival ninja clans, or the Mahoroba butterfly-demon tribe, Yaku consistently fights alongside Miko as a resilient and capable warrior. Her core traits, abilities, and role within the Miroku clan's conflicts remain unchanged across official media appearances, including the original "La Blue Girl" OVAs and the sequel "La Blue Girl Returns." Maria, the human queen of the Shikima realm and Miko's mother, is a separate character with no alias or alternate identity connection to Yaku.