Princess Serenity

Princess Serenity was the real name of the Moon Princess. She was the previous incarnation of Usagi Tsukino, as was revealed part-way through the storyline of the first season. She was the crown princess of the Silver Millennium, but she showed little interest in her royal duties as she was infatuated with Prince Endymion of the Earth Kingdom.

Manga
In the manga, contact between the two kingdoms was permanently forbidden for unknown reasons. However, the Princess would often travel to Earth to be in Endymion's arms. This Serenity was shown in flashbacks to be almost as childish and enthusiastic as her reincarnated self.

When Queen Beryl killed Endymion in front of the Princess' eyes during her invasion of the Silver Millennium, Serenity took her own life with his sword. In the present day, Princess Serenity awakened after Mamoru Chiba sacrificed himself to save her from a deadly attack.

Anime
In the anime, Prince Endymion would sometimes visit the Moon Kingdom to see Princess Serenity. One day, not long before a planned attack on the Moon Kingdom was about to take place, Endymion visited the Moon Kingdom to warn Princess Serenity. Guards saw him and tried to chase him away. At the ball, the Prince appeared to greet the Princess at the staircase in disguise wearing a tuxedo and mask. He told the Princess that danger was coming, but he lingered with her too long and got caught up in the battle.

When he tried to defend Princess Serenity from Queen Beryl, the two were separated by a gust of wind which pulled the Prince up towards Queen Beryl. When the Princess jumped to reach her love, Queen Metalia killed both of them with an energy blast.

In the 20th century, Usagi Tsukino was revealed to be the reincarnation of Princess Serenity in episode 34. In episode 46 she appeared to defeat Super Beryl, and Usagi did not appear in that form again until episode 91, when she received the Spiral Heart Moon Rod. Her final appearance was in the final episode, where she battled against Sailor Galaxia.

Pretty Guardian Sailor Moon
In the live-action series, Princess Serenity's personality was more thoroughly examined and extrapolated on. She was revealed to have been cold, childish, and in some ways selfish. In this version, the destruction of the Earth Kingdom and Moon Kingdom was caused by her, in her sorrow over Endymion's death.

After the Princess within Usagi was awakened, she began to slowly take over, and took control of Usagi's body when she was suffering from sadness, loneliness, or anger. When Mamoru was threatened, Usagi would transform into Princess Sailor Moon against her will.

According to Minako, Serenity used to be a kind and peaceful girl, much like Usagi in her present incarnation, but that changed after the destruction of the Moon Kingdom and the Earth Kingdom, making her cold and ruthless and always believe that her and Endymion's destiny was only to separate, even if they were born on the same planet, Earth.

Like the live-action version of Queen Serenity, Princess Serenity did not wear the odango hairstyle. Her dress was also different than that of her anime and manga counterparts.