Serenity was the queen of the Moon Kingdom 1,000 years ago (around 900 AD) during the Silver Millennium. She had one daughter, Princess Serena, who she was raising to follow in her footsteps to one day rule her Kingdom. Serenity was a peaceful, loving, and wise ruler. She cared so much for her daughter and gave up her own life, by using the Imperium Silver Crystal to save her, and the rest of the people on the Moon.
Her daughter, Princess Serena, was later reborn as Serena Tsukino on Earth in the present and became the champion of Justice, Sailor Moon. The Queen reappeared in spirit form and revealed the past to the Sailor Scouts. She also upgraded Sailor Moon's Sailor Locket to the Crystal Star Locket.