The Wicked Lady
The Wicked Lady is created just before the death of Sapphire, when the Wise Man finally gets his hands on Chibi-Usa/Rini. He uses his powers to warp her thoughts and fill her with the energy of the Black Crystal, which produces the familiar pink-haired adult woman with the evil look (the aging effect is a perversion of her wish to be a beautiful lady like her mother). Anyway, Chibi-Usa is the only one who can withstand channeling the necessary amount of power from the Black Crystal to open the Black Gate, and she therefore becomes a valuable asset to the Wise Man, who wants to use that power to open the gate and destroy the world. Presumably, he first notices the potential of Chibi-Usa's unusual power when Emerald uses Chiral and Achiral to attack the school she's attending. Chibi-Usa becomes upset when her friend Momo is attacked, and she lets loose with an incredible burst of energy, prompting the WM to change their plans. Instead of capturing Chibi-Usa to kill her or to get the Silver Crystal, the clan (or the Wise Man) will capture Chibi in order to exploit her incredible power for their own ends. Chibi is eventually captured and has her memories manipulated so that she will believe that everyone who loves her actually hates her. Sailor Moon has a devil of a time trying to break the Wise Man's spell once she figures out that the Wicked Lady is actually Chibi-Usa and needs to be saved. Chibi-Usa remains the Wicked Lady until she tries to open the Black Gate. Before she can open it all the way, Sailor Moon manages to purify her and turn her back into Chibi-Usa. It's then revealed that the Silver Crystal that she'd misplaced originally was inside of her body the whole time (she cries it out in one of her tears). Sailor Moon and Chibi then combine the powers of both Silver Crystals to destroy the Doom Phantom and the Black Gate.
Since she has essentially been brainwashed, the Wicked Lady is completely loyal to the Wise Man. Whereas by this time, everyone else seems to be doubting his worth, the Wicked Lady will do whatever the Wise Man asks her to. As a result, she is extremely ruthless. The Wicked Lady also strikes me as being a bit seductive. She likes to flaunt herself a little and she doesn't seem to mind too much about the high slit in her dress, even though most of us would find it a little, um, embarrassing. Anyway...The Wicked Lady is pretty much the same in the manga, but she is also referred to as the Queen of Nemesis and she sits with the Wise Man at the end of the 4th dimension. Eventually, the Wicked Lady comes to Earth and throws some poison crystals around, and she begins to open the Black Gate. At the same time, Diamond somehow gets his hands on both Sliver Crystals and is about to put them together, which would destroy the universe, when Sailor Pluto stops time. Stopping time kills her, however, and when Wicked Lady sees that her only friend is dead, she begins to cry and is turned into Chibi-Usa again.
An interesting note: In the original Japanese version of Sailor Moon R, the Wicked Lady is actually called the 'Black Lady'--this was changed in North America for obvious reasons. She gains the designation 'Black' in more of a literary sense, in that the color black has traditionally been associated somewhat with darkness, death, and evil. The designation also attests to the fact that in her transformation, she has become a lady of the planet Nemesis, which is also known as the Black Moon. So she is the "Black (Moon) Lady".
Wicked Lady Pictures: