Nagisa Mikogami, the eldest sister to twins Riko and Rimu, possesses brown hair and purple eyes and attends middle school. Her personality reflects maturity and responsibility; she acts protectively during her sisters' adventures. Nagisa demonstrates attentiveness to parental guidance and assumes duties like knowing the location of confiscated Kirumin transformation devices, uncovering family secrets such as Doctor the turtle's true identity as their grandfather Yuuki Mikogami.

Her primary Kirumin transformation is into a dog, matching her fondness for canines. This form grants an enhanced sense of smell used to track missing animals or people. As a Detective Club member, she collaborates on animal-related mysteries, applying her cautious nature to balance other members' impulsiveness. Like her sisters, Nagisa is one-quarter Animalian, though her specific Animalian transformation remains undisclosed.

Her relationships include a close friendship with Kasumi Miyabe and romantic feelings for Pars Ryūdō, a wolf Animalian affiliated with Misa Hatori's group. This connection occasionally distracts her during missions with objects related to Pars or Kanon Hatori. Nagisa mediates between her sisters' energetic pursuits and parental expectations, particularly when Riko and Rimu take risks.

Throughout the series, Nagisa exhibits gradual development in leadership and responsibility. She aids in resolving conflicts involving Animalians, including Futatsugi's antagonistic schemes and reconciling Misa Hatori's group with the Mikogami family. Her heritage is clarified as part Animalian through her grandmother Marianne Schonberg, making her a cousin to Kanon Hatori. In the finale, she participates in thwarting Futatsugi's plans and witnesses her grandfather's restoration to human form.