When the Senshi first met Queen Beryl, she was alarmed to find out they were originally from the Moon. This frightened her that her mother wasn't really her mother. Luna seeing how disturbed Usagi was and reassured her that her mother was really her mother. She eventually began to warm up to Mamoru, whose path kept crossing with hers. Little did she know, he discovered she was Sailor Moon in a mirror house. On Valentine's day, she made a muffler she wanted to give Mamoru but was sadden to find out he was engaged to be married to a childhood friend named Hina. Even worse, Hina was nice and sweet and Usagi couldn't find herself hating her love rival. She eventually discovers that Mamoru and Tuxedo Kamen are the same person, much after the others did. The Senshi believed their mission was to find and protect the Princess. When Zoisite attacked them, Sailor Venus revealed she was the Princess. They all found they were mistaken, when Usagi was healing the wound Tuxedo Kamen and changed into the Moon Princess. After this, Hina breaks up Mamoru and has him travel to London to study without her.
Long ago, Usagi was Princess Serenity of the Moon and Mamoru was the Prince Endymion of Earth. Their love ended in tragedy and was the cause of the end of the Silver Millennium, a time of peace. When Mamoru returns from London, he tells her he returned to prove that would never occur again. He then asks her for the muffler she made for him. Sailor Venus and Zoisite even teamed up to split the two up. Zoisite gives Sailor Venus a music box in exchange for Venus protecting Endymion since Beryl had a tight grip on them both. Usagi sung Minako's song "C'est la Vie", and Minako realizes that the Princess truly does love Endymion, and drops the music box, allowing it to shatter on the floor. When Beryl tried to convince Mamoru, to come to the Dark Kingdom, Princess Serenity resurfaced within Usagi and went on a rampage. Whenever Serenity used the Ginzuishou, Metalia's power somehow increased. With Endymion trapped in the Dark Kingdom and in danger, Usagi could barely keep Serenity's power suppressed. And when Mamoru took Metalia within himself, Usagui had to no choice but to kill him. When she realized what she had done, Princess Sailor Moon took over Usagi and released a wave of total destruction from the power of the Ginzuishou.
Endymion appears before her in a desert and she rushes to him, upset because she was unable to keep her promise not to destroy the world. He tells her she can use the Ginzuishou's power to save the world. She agrees, even though it might cost her life. He wonders if this is what they need to do in order to bring an end to the past life. He apologizes for not being able to save her. Endymion slides a ring onto Serenity's finger and kisses her. The Ginzuishou restores the world, leaving the Prince and Princess lying lifeless on the sand. Life goes back to normal for the Guardian Senshi, but they felt something was missing. Princess Sailor Moon appeared to Usagi and told her that as long as her friends remember her, she can live. The Ginzuishou left her life behind, even though it shattered. At last the past has ended. Mamoru tells Usagi to go on and forget about him, but she refuses. Endymion's guardians, the Shitennou's wish also brings Mamoru back. In the Special Act, four years had passed and Usagi and Mamoru are to marry but after an argument, Usagi returns the ring and cancels the wedding. She and Mamoru got kidnapped by Queen Mio and Usagi managed to escape, at Mamoru's orders. Mamoru was hypnotized to marry Queen Mio, but Usagi, looking at her engagement ring that Mamoru slipped in her pocket, she regained her compsure and went to get Mamoru back. Mio gave Mamoru a sword to kill Usagi, he threw it and she didn't flinch or move, it went passed her. He expressed that she scared him by not moving, she explained she always know when he is being serious. Usagi and Mamoru married after Queen Mio was defeated. Both Makoto and Motoki mentioned that the wedding seemed rushed. Before the wedding, Usagi seemed about to vomit and ran to a bathroom.