Designated DC138621-S113, D is an operative for the Lepton Kingdom of Alpha Cygni, an all-female alien race. Stationed long-term on Earth, her primary mission is locating their missing princess, C-ko, who crash-landed as an infant. This assignment culminates when D identifies C-ko attending Graviton High School, prompting coordinated retrieval efforts with her commanding officer, Captain Napolipolita. Despite her dedication, D suffers repeated severe accidental injuries during encounters with A-ko and C-ko, often collateral damage from A-ko's superhuman speed and strength.

Physically, D presents a masculine appearance by human standards, typically wearing a trench coat and sunglasses, leading Earth observers to misgender her. This confusion resolves when she dons a revealing uniform during a confrontation, explicitly confirming her biological sex aligns with her all-female race. Her design intentionally parodies the character from *Vampire Hunter D*.

Following the first film's events, D and Captain Napolipolita become stranded on Earth after their spaceship crashes and balances precariously atop Graviton City's Military Command Tower. In *Project A-ko 2*, D experiences profound homesickness and participates actively in the aliens' efforts to repair their vessel. These efforts include converting the damaged ship into a temporary hotel complex to raise funds. D remains a loyal subordinate to Captain Napolipolita, supporting their shared goal of returning home, ideally with Princess C-ko. Her mission is complicated when military forces, manipulated by B-ko's father, Hikaru Daitokuji, attack the ship to seize its advanced technology, forcing D to defend against these incursions.

D's character is defined by unwavering duty to the Lepton Kingdom, resilience in the face of repeated physical mishaps, and adaptation to prolonged Earth exile.