Sanae Nakajima, a diligent high school class representative, shares a close bond with Kōshi Inuzuka while secretly operating as the superhero Tenchū Senshi Uma Kamen ("Divine Punishment Warrior Horse Mask"). This dual identity stems from her lineage in the Nakajima clan, guardians linked to the Horse of the Chinese zodiac and sworn to protect the Inuzuka family heir—a duty upheld by generations, including her grandmother Kimie, the former Uma Kamen and current landlady who embodies the clan’s martial heritage.

The Nakajima combat style revolves around devastating kicks, demanding specialized attire that amplifies strength through an unconventional design: removing pieces heightens power concentration on exposed skin, channeling greater force. Maximum efficiency requires Sanae to wear only the boots during battle, a humiliating necessity clashing starkly with her reserved demeanor.

Her role as Kōshi’s protector intertwines painfully with clandestine affection for him, emotions she stifles to honor his arranged engagement to Momoko Kuzuryū. This turmoil intensifies when pressured by her grandmother to marry Ikaru Nakajima, a socially inept clansman. Sanae defies tradition by defeating Ikaru in combat, securing independence without abandoning her duties.

Navigating her dual life, Sanae supports Kōshi and Momoko academically and emotionally, mediating disputes or aiding them in conflicts. Her quiet resilience prioritizes their safety and harmony, reflecting selfless loyalty to clan traditions even as she privately grapples with sacrifice. Over time, she reconciles her obligations with personal yearnings, growing into her role without compromising integrity.

The Nakajima legacy emphasizes subtlety over overt aggression, mirroring Sanae’s understated nature. She safeguards Kōshi discreetly, shielding her ordinary school persona while embodying steadfast resolve. Her journey highlights the quiet strength of a guardian who upholds duty amid emotional complexity, anchoring stability within the chaos surrounding her.