Malachite was one of Queen Beryl's four generals; the fourth to be specific. His job, initially, was to aid Zoisite in claiming the Rainbow Crystals for Queen Beryl, which would allow Beryl to re-create the Imperium Silver Crystal. Regrettably, Zoisite had a vindictive streak in her, which would lead to her doom. Zoisite, after vengeance, tried to kill Tuxedo Mask, with whom Beryl had fallen in love. Her reasoning was that with all seven Rainbow Crystals (which Zoisite would gain by killing Tuxedo Mask) Beryl would forgive her. Unfortunately, Zoisite did not get Tuxedo Mask's crystals, and Beryl was rather unforgiving. Her failure cost her life, and cost Malachite his true love. Malachite was, besides Zoisite's helper, her trainer and her love. All Zoisite wanted as she died was to be with Malachite, which she was. She died in Malachite's arms, about as romantically as death can ever be.
After this tragic loss, Malachite was Beryl's only remaining subordinate and he took Zoisite's place. Malachite's death was soon to follow however. Malachite was the only general to die as a direct result of an action of one of the Scouts. He was attacking them with what look like boomerangs of energy, and would likely have won, but Sailor Moon deflected one of Malachite's own attacks which struck him on the rebound, killing him. His dying words were, "Zoisite, its me! I'll be coming to join you real soon, Zoisite!. Do you hear me?!". His last thoughts were not of his death, his failure, or his hate for those who had brought him down, but of the reunion with Zoisite to which he looked forward. Evil or not, Malachite was as loyal in love to Zoisite as he was in duty to Queen Beryl.
In the Japanese version, Zoisite was actually a male. They changed it for the US because they didn't think a gay relationship was appropriate for US viewers.