Marcia the Martian is Marvin the Martian's apprentice and niece; as such she is also a Martian. She only appeared in Duck Dodgers Jr., where she went up against Daffy and Plucky. At the end she and Plucky decided to go off and play, while Marvin and Daffy duke it out.
Marcia is about 5 years old. She enjoys playing with building blocks, and is a very fast speaker. She is playful, and loves making friends. She befriends Plucky Duck (despite him being considerably older, and being the biggest fan of her uncle's arch enemy Duck Dodgers.) She only appears in one episode, but is well liked. Marcia is very girly. She wears a pink jumper, and trousers with a purple skirt and cone party hat that has a pink bow. She has sneakers which resembles her uncle's. She is supportive of her evil uncle for the simple reason that she loves him. She is often mistaken for Marvin's daughter as she calls him "daddio" in the episode.
It is unknown whether Marvin has a brother or sister, because Marcia's parents aren't seen. It is still obvious that Marvin is the youngest of whatever siblings.
Her name, after considerable study, appears to be a corruption of the Koshallic name Ma'rwasth'eea, or "Female Spirit of the Double Moonrise". Strangely, she gained her adult name approximately eight Earth years before the general ceremony. However, given the somewhat eccentric nature of her family, it shouldn't be too difficult to allow for extenuating circumstances.