Mio Kuroki (黒木ミオ Kuroki Mio), played by Alisa Yuriko Durbrow, is a strange and manipulative girl who joins Usagi's class at school. She is a fellow pop idol and thus has a rivalry with Minako Aino. She pretends to be Usagi's friend while doing all in her power to cause her sadness. Because of her forgiving nature, however, Usagi continually gives Mio extra chances. Mio soon becomes very envious of Usagi's closeness to Mamoru and fakes fainting in front of his motorcycle in order to kidnap him and take him to Queen Beryl. There, Mio taunts Mamoru, telling him that Beryl will kill him if he does not join the Dark Kingdom and become Beryl's lover.
Mio is revealed to be created from a part of Queen Beryl herself, apparently to do harm to Usagi. After kidnapping Mamoru, her main role is to keep an eye on him for Beryl. The only powers of her own she demonstrates are slight mind control powers, and teleportation, of both herself and others.
Toward the end of the series, Mio is apparently killed by Endymion/Metaria; she is somehow resurrected, however, as the main villain of the "Special Act". Once again, she kidnaps Mamoru, this time wanting to marry him so they can rule a new Dark Kingdom as King and Queen. She is confronted and attacked by the Shitennou, forcing her to transform into a plant-like monster that makes quick work of her former allies. Ultimately, her final form is destroyed by the Senshi's combined powers, channeled through the Moon Stick in the Sailor Planet attack.
After her revival for the Special Act, she seems to gain new powers. She can repair objects, cause them to appear out of thin air, and create monsters from flames. She can also summon legions of pierrot, or clowns, to serve her and gather energy, even creating replicas of the Shitennou. Mio is also able to brainwash others and transform into a plant-like monster. At one point she interrupts a television broadcast to announce that she is taking over.