Natsuki Saotome, the seventh of nine sisters, serves as a Valkyrie dispatched from Asgard to defend Midgard against demonic threats. She possesses long, wavy blond hair, blue eyes, and often wears a butterfly-wing headband in her human form, adding red glasses during study sessions. Her Valkyrie battle form features a black uniform woven with golden threads, a black tie bearing red stripes, her hair gathered into a side ponytail, and a distinct cross pattern visible within her eyes. She wields the Holy Sword "The Blade," enabling her to manipulate multiple large blades in combat.

Initially displaying a strong-willed and popular personality among classmates yet unaffected by her status, Natsuki harbors no early attraction towards Takuma Akutsu, the Einherjar assigned to support the Valkyries. She gradually develops respect for his persistent efforts, his dedication motivating her to strive harder in battles and fostering a mutual bond. She exhibits Brutal Honesty, delivering blunt remarks that visibly impact others. Her character arc includes moments of emotional vulnerability, such as feeling hurt when Takuma declines her dance invitation at a school festival due to social anxiety, though he later rectifies this.

Her relationship with Takuma evolves significantly. Early on, she challenges his reluctance to engage in the Valkyries' mission, criticizing his skewed priorities regarding studying versus saving the world. She plays a pivotal role in his emotional growth, helping him transition from reclusiveness to proactive participation. Their intimacy progresses through "Val Love" events, including a critical Level-Up ritual requiring five minutes of uninterrupted kissing while nude, which unlocks her enhanced combat abilities. This culminates in romance: Takuma reciprocates her love confession with a marriage proposal, recognizing her as his foremost emotional attachment despite his connections to all nine sisters. The series concludes with their marriage and the birth of their child.

In combat, Natsuki holds the distinction of being the highest-leveled Valkyrie among her sisters. Designated Valkyrie of the Blade, Siegrune, she specializes in wielding and telekinetically controlling multiple blades via her Holy Sword. Her power directly correlates with her emotional bond to Takuma, strengthening through physical and romantic intimacy. Key battles involve collaborative efforts with her sisters to defeat demons during school festivals and other incursions, where her upgraded abilities prove crucial after successful Val Love rituals.

Her background includes a name reflecting her birth order: "Natsuki" combines the characters for "seven" (七) and "tree" (樹), while "Saotome" signifies "early maiden." Residing on Earth under Odin's command, she balances high school life with Valkyrie duties, frequently participating in events like maid cafes or festivals to facilitate Val Love activities. Her narrative spans the entire series, from initial missions to the final battle against apocalyptic threats, solidifying her role as both a warrior and Takuma's primary romantic partner.