Castor and Pollux are twin female youma under the control of Nephrite in episode 021 (Protect the Childrens' Dream! A Frienship Tied by Anime). Because they were Nephrite's strongest youma, their primary mission is to help him defeat the Sailor Senshi. The other part of their mission is to gather the energy of the young animator Hiromi Matsuno once she reaches her peak energy output. Nephrite puts his evil crest on one of Hiromi's animation pencils, causing her to become obsessed with drawing better images than her friend Kazuko Tadashita, and hastening her approach towards her peak energy output. Once Hiromi reaches her productive peak, Castor and Pollux immediately emerge from the pencil and drain all of the girl's energy. Kazuko, Usagi, Ami, and Rei are all on the scene. Kazuko faints, and the girls take the opportunity to transform and challenge the youma. The youma say that because they are bound together by friendship, they cannot be defeated. Castor attacks the senshi with a fiery attack like Sailor Mars's Fire Soul. Next, Pollux attacks with an attack like Sailor Mercury's Sabão Spray. The senshi are confused because the youma are using attacks very much like their own. The youma step forward and Castor says that she will finish the girls off. Pollux angrily says that that is her job! The two youma immediately begin to argue, and decide to end their friendship. Their tails, which had previously been tied together, break apart. The senshi attack with a powerful combination attack: Sailor Moon uses Moon Tiara Action, Sailor Mars uses Fire Soul, and Sailor Mercury uses Sabão Spray. As the tiara flies through the air, it is surrounded by flames and bubbles. It hits the youma, who say that they should have valued their friendship more. They are instantly destroyed, and Hiromi's energy is returned.
Castor and Pollux are identical except for their coloring. They both have long tails which are tied together at the end. Castor has orange skin, red eyes, light brown hair, and a red forehead marking. She wears a magenta outfit. Pollux has blue skin, blue eyes, light blue hair, and a yellow forehead marking. She wars a turquoise outfit. Castor's seiyuu is Akiko Hiramatsu, and Pollux's seiyuu is Yuka Ohno.
Castor and Pollux are named after two stars, which are named after mythological characters. In astronomy, Castor and Pollux are the two brightest stars found in the constellation Gemini, which Nephrite says points towards Hiromi Matsuno. These stars are sometimes called "heavenly twin" stars, although they are really nothing alike. In Greek mythology, Castor and Pollux were two twin sons of Leda and Zeus (who seduced Leda in the guise of a swan). In many versions of their story, Castor was mortal while Pollux was immortal. When Castor died, Pollux begged Zeus not to separate them, and the twins were given the ability to alternate their mortality: they would spend one day as gods, and the next as deceased mortals in Hades.