Prince Diamond is the leader of the Negamoon Family, and the main villain of Sailor Moon R. He is in love with Sailor Moon, while Emerald is in love with him. His name is based off the gemstone Diamond. He is Prince Sapphire's older brother.
Prince Diamond was among those banished to Planet Nemesis after the attempted invasion of Crystal Tokyo. So he attempted to get revenge by attacking Crystal Palace, but found it to be invincible, so he sent Rubeus into the past to take over the crystal points in Tokyo, and also to find Rini, who also traveled into the past with the silver crystal.
After the death of Rubeus, Diamond made his first appearance at the end of Episode 27 (though he was mentioned in the previous episode). He sent a droid named Grim Man into Rini's dreams, but Grim Man was destroyed by Sailor Moon.
Afterward, Emerald devised a plan to take over Tokyo, but her plans were foiled by the Sailor Scouts. Diamond called Emerald back to the future after a while.
When the Sailor Scouts traveled to the future, Diamond tried to hypnotize Sailor Moon into loving him, but she escaped with the help of Tuxedo Mask.
Diamond was so in love with Sailor Moon that he did not realize how volatile or fragile the dark crystal was, and that putting new energy into it could make it explode. His adviser, Wise Man, clearly did, and Diamond's younger brother Sapphire suspected him.
After Emerald's death, Wise Man turned Rini into Wicked Lady. Diamond traveled into the past with Wise Man, Wicked Lady, Sapphire, and the Dark Crystal. Soon Sapphire discovered Wise Man's true intentions, confirming his worst fears. He tried to warn Diamond, who initially did not believe him, but, against Diamond's orders, Wise man killed his younger brother Sapphire first right in front of him. From here, Diamond turned on Wise Man and said he would destroy him after he took over the world, unaware that Wise Man's power was greater than his own.
Diamond unsuccessfully attempted to hypnotize Sailor Moon again, and he soon realized that Sapphire was telling the truth, and Wise Man was manipulating him all along (their original goals did not even involve destruction). Diamond fought Wise Man to avenge Sapphire and Emerald's deaths, but Wise Man directed an attack at Sailor Moon. Diamond jumped in front of her and was impaled by the dark spears. He then blasted Wise Man, seemingly killing him. However, as Diamond was dying in Sailor Moon's arms, Wise Man came back together.
- Diamond can shoot a white energy ball, a white energy beam, or hypnotize people with the third eye on his head. This eye only ever appears to hypnotize.
- Diamond is the third villain to be redeemed and die, though unlike Neflite, it was Wise Man who betrayed him.
- On that note, he is also the third redeemed villain to be stabbed in bloody fashion, first being Neflite and second being Ann (blood is seen trickling from her mouth after the stabbing), although his death was less graphic than that of Neflite. It is interesting to note that the bloodier deaths of Sailor Moon seem to go to the redeemed villains.
- Prince Diamond's name is similar to character-of-the-day Princess Diamond from Sailor Moon (except he's a prince as opposed to a princess), who is completely unrelated to him in any way.
Voiced by Robert Bockstael