D is a humanoid alien and one of the core robot pilots for the Earth Defense Force in the parallel world. Her true origins lie with a colony that came to Earth thousands of years ago, from which she fled after a catastrophic conflict claimed the lives of her friends. She escaped in the original core robot named Zinv, seeking to evade the malevolent alien presence known as Kumu and the robot Himiko. When the alien artifact containing her and the robots was discovered in the parallel world, her original body was beyond repair. Scientists managed to preserve her essence, her astral self, by placing it into a series of artificial bioroid bodies. The fourth model was the first successful one, which is why she is called D. In this body, her astral form is visible as a glowing orb that takes the place of her left eye.

D possesses a detached and stoic demeanor that makes her seem as mechanical as the robots she pilots. She speaks in a flat, almost robotic tone and asks frequent, direct questions about human emotions and social behavior, which she struggles to understand. This artificial exterior hides a vulnerable and traumatized past, as she is haunted by recurring visions of a horrible creature and lives in genuine fear of Himiko. Despite her alien intelligence, her emotional development is stunted, and she relies on the bioroid body to exist on Earth, as her original form cannot survive in the planet's environment for long.

Her role in the story is closely tied to her discovery of human connection. Commander Ken Sanada, the head of the Earth Defense Force, arranges for her to live in the same house as the protagonist Kazuki Yotsuga, believing that exposure to him will help her experience new emotions. This proves to be accurate, as Kazuki is the only person who manages to break through her stony exterior. Her key relationship is with Kazuki, to whom she develops a strong and devoted attachment. Through her interactions with him and the others in the household, she slowly begins to learn about friendship, jealousy, and eventually love.

D is not a particularly skilled pilot and frequently requires rescue by Kazuki or Mitsuki Sanada during combat. However, her true value lies in her connection to the ancient technology. In the climax of the series, her deepening feelings for Kazuki lead her to make a critical decision. She chooses to return to her original, long-damaged body in order to save him. In this restored form, she is able to remove the limiter placed upon Zinv, unlocking its full, devastating power. After Zinv is destroyed and the parallel worlds merge into a single new reality, D is given a new chance at life. She is transformed into Dee Sanada, a seemingly normal and animated young girl who becomes the adopted daughter of Ken Sanada, having finally gained a stable body and a newfound capacity for joyful, human expression.