Zoisite was the third of Beryl's minions to confront the Sailor Scouts. She was evil and backstabbing (what more could you want from a villain?). She actually killed her predecessor (Nephlyte) to gain his position and Queen Beryl's favor. Her mission was to gather the seven Rainbow crystals which were scattered throughout the world and embedded in unsuspecting humans. She used Nephlyte's dark crystal to activate the rainbow crystal in her victims, thus turning the humans into negaverse servants. The only cure for this was Sailor Moon's Lunar wand (she had to use ' Moon healing activation' ).
Zoisite was Malachite's pupil and lover. They both had a pure hatred of the scouts and Tuxedo Mask, but she really hated Tuxedo Mask while Malachite despised Sailor Moon. Eventually she teams up with Malachite against the Scouts forming a Very formidable combination. She too dies, but this time at the hand of Queen Beryl... she failed one too many times. In my opinion she deserved it for killing Nephlyte.
Just a tidbit... In the Japanese version Zoisite is a male (you do the math!)