Prince Diamond was the ruler of the Negamoon. He was the older brother of Saphire too. The two brothers were close when they were children but through the years, the Wiseman used Diamond and corrupted his mind which definately put a damper on the two's relationship.

One of his biggest weaknesses was that he was obsessed with Neo-Queen Serenity, which ends up being his end. Emerald was the one in love with him, but he was too busy taking ver Earth and lusting over Sailor Moon to realise it. In the end he died saving Sailor Moon from the Wiseman.