Prince Diamond was the leader and the most powerful member of the Black Moon Clan (not counting Wise Man as part of the clan). He had devoted himself to seeking vengeance against the Earth for his clan's exile from 30th century Crystal Tokyo. He was also attracted to Neo-Queen Serenity and had his mind set on making her his.
Prince Diamond acted under Wise Man's guidance the whole time. Prince Diamond loved his younger brother Saphire. He tried to stop Wise Man from killing him but Wise Man did not listen. He has since grown wary of Wise Man and promised himself that he would settle the score of his brother's death after his revenge was complete.
Prince Diamond soon realised that Sailor Moon was the woman he coveted in her younger days. He succeeded in catching her a few times and tried unsuccessfully to make her his... (Tuxedo Mask would always save her at the last moment...).
With Sailor Moon's help, Prince Diamond was able to see the truth in the end -- Wise Man had been using the whole Black Moon Clan for his own purposes the whole time. Wise Man commanded Prince Diamond to kill Sailor Moon but he refused. Ultimately, he and Wise Man fight each other. He could have won but he sacrificed his life to save Sailor Moon. With his last breath he tells Sailor Moon to take care of the Black Moon sisters living on Earth, and that he is glad there is still hope for his clan.
