D

Stationed on Earth for an extended period as a spy for the Lepton Kingdom, D has been searching for traces of their missing princess. Her long exile bears fruit when she spots A-ko and C-ko rushing to school one morning. She persists in tailing the duo, even though she sustains severe (yet accidental) physical injuries every time they cross paths.
Much like the Captain, D is actually female, though she presents a very masculine appearance to Earthlings. This misunderstanding is resolved at the conclusion of the first movie, when she wears a very revealing uniform to fight A-ko.
While D does attempt to kill A-ko, she stays fiercely loyal to C-ko, even going so far as to taste C-ko's poisonous cooking out of respect. Nevertheless, her primary objective is to bring C-ko back to the Lepton Kingdom, regardless of whether C-ko wishes to return. Throughout the series, she is seen staying close to Napolipolita, suggesting they are close friends as well as fellow soldiers.