As a spy for the Lepton Kingdom, D has been stationed on earth for a long while, searching for signs of their missing princess. Her long exile comes to fruition when she encounters A-ko and C-ko racing to school one morning. She continues to stalk the pair, despite suffering massive (but accidental) bodily harm whenever they meet.

Like the Captain, D is in fact a woman, but appears very masculine to Earthling eyes. This confusion is cleared up at the end of the first movie, when she dons a very scant uniform to battle A-ko.

Although D does try to kill A-ko, she remains fervently loyal to C-ko, even going so far as to sample C-ko's toxic cooking out of respect. However, her main goal is to get C-ko back to the Lepton Kingdom, whether she wants to return or not. During the series, we see her sticking close to Napolipolita, indicating they are close friends as well as fellow soldiers.

(Source: Wikipedia)