Usagi and Mamoru are vital to the main plot as the protagonists of the story. Without their doomed relationship within the time of the Moon Kingdom, Sailor Moon as we know her would simply not exist and there would be no tale to tell. It is the love story between Serenity and Endymion which kick-starts the rebellion of the Earth Kingdom and the corruption of Queen Beryl. This rebellion takes the lives of both Serenity and Endymion, leading to their reincarnation in modern day Tokyo and the beginning of the new battle with the Dark Kingdom which is where the story starts for the viewer.
Upon their first meeting Usagi and Mamoru don't get on quite as destined lovers should. Usagi has just failed a test with 40/100 and in despair throws her scrunched up test paper behind her. Naturally, it hits Mamoru in the face leading to a brief confrontation. She believes him to be stuck up and he believes her to be stupid, having seen her test result. He compounds her dislike of him by referring to her as "Odango" or "dumpling"-head. This makes her angry and their relationship is soon set as one of annoyance and intense dislike.
However, Mamoru is also Tuxedo- Kamen who, as early as the first episode establishes himself as Sailor Moon's love interest. It is later in the first series, when the location of the Silver Crystal is revealed that Tuxedo-Kamen's identity as Mamoru Chiba is uncovered, much to Usagi's dismay. It is also important to note that with the Silver Crystal, the memories of the Senshi and Tuxedo-Kamen return to them and Usagi and Mamoru recognise each other truly for the first time. The joy of this moment is cut short as Mamoru is kidnapped by the Dark Kingdom and Brain washed. this opens up a whole new stage in the relationship between the couple.
The Brain-washing of Mamoru allows Usagi's character to expand and mature. For the first time she does not have Mamoru to save her anymore and also for the first time she knows and accepts her love,which makes her seperation from Mamoru doubly difficult. In the anime, she heals him and saves him however in the manga, the story takes a far more dramatic turn. Usagi, destitute with grief at her inability to save the man she loves, resolves to at least save him from himself and the taint of association with the Dark Kingdom. She takes the Silver Crystal sword and kills him, before killing herself. This is a strong statement by Naoko Takeuchi about the strength and depth of Usagi's feelings for Mamoru and it allows the audience to see a different side of Usagi's usually playful and relaxed character. With the help of the Silver Crystal, Usagi and Mamoru are healed and go on to defeat the evil Queen Metallia, leading to a short respite of peace.
In the anime however, there is yet more seperation for Usagi to endure. After a small time during which Usagi and Mamoru are happy and in love, Mamoru begins to have dreams informing him that if he stays with Usagi, then disaster will befall her. He breaks up with her and much of Sailor Moon R is spent dealing with the emotional anguish that this causes them both.
In the live action version of Sailor Moon, the plot line between Usagi and Mamoru is slightly different. She finds out that he is the engimatic Tuxedo Kamen but also that they cannot be together as he is already promised to another young woman. This story emulates the pain of seperation shown in the R season.
Eventually, Endymion and Serenity have their happy ending and are married. Naoko Takeuchi always swore to herself that the story would end with their wedding, and so it did.
