Miki Shirasagi attends the same school as Asuta Jimon and Renge Komadori, distinguished by elegance, intelligence, and athletic prowess that cement her status as a model student admired by peers. Her background stems from the prominent Shirasagi family, whose wealth and influence manifest in her refined demeanor, blonde hair, and a distinctive beauty mark under her eye. She shares a longstanding acquaintance with Asuta Jimon, rooted in their families' childhood connections. A notable incident from their past involves her proposing to him as a child, which he immediately rejected, creating enduring awkwardness still referenced among schoolmates.

Secretly, she operates as Sergeant White Egret within the justice organization White Light, opposing the world-conquest ambitions of the Zvezda group. In this role, she demonstrates exceptional combat proficiency, engaging multiple adversaries simultaneously—including formidable opponents like Itsuka Shikabane and Goro Shikabane. While respecting Renge Komadori (White Robin), Miki occasionally exhibits a teasing or enigmatic attitude toward her, describing Renge as possessing a "pure heart worthy of being followed." Her secret identity is dramatically revealed during a school trip when she openly removes her mask.

As events unfold, White Falcon promotes her to White Light's new commander, leading to collaboration with the Tokyo Army. This position coincides with a shift in her demeanor: she grows more reserved and self-centered, particularly as her authority expands, yet maintains resolve in critical moments. Ultimately, she allies with Renge Komadori to defect from White Light and join Zvezda, marking a pivotal turn in her character arc and the overarching conflict.