An operative of Alpha Cygni's Lepton Kingdom designated DC138621-S113, D undertook a prolonged Earth mission to locate the lost princess C-ko. D's notably masculine appearance frequently causes Earth residents to mistake this member of an all-female alien species for male, fueling narrative confusion. Typically conducting surveillance on A-ko and C-ko while wearing a trench coat, D routinely suffers significant accidental physical harm during unexpected encounters with the protagonists. After the resolution of the initial alien conflict, D is often seen accompanying Captain Napolipolita. In Project A-ko 4: Final, D's role diminishes; appearances are primarily alongside the Captain, with minimal involvement in the central wedding plot or large alien fleet arrival. Professional frustration marks D's experience, stemming from the Captain's unreliability and the challenges of extended undercover Earth work. During earlier action sequences, D adopts a distinctive, scantily-clad battle outfit, contrasting sharply with the usual concealed look. The character's design and narrative function incorporate parody elements specifically referencing the protagonist of Vampire Hunter D. Official media documents no substantial character evolution or expanded background beyond these operational and comedic elements.