Chiba Mamoru, or his alter ego Tuxedo Mask, is a dedicated friend of the Sailor Scouts.
In the past, Mamoru was Prince Endymion of Earth, so in a way, he is the sailor Earth. He was in love with Princess Serenity of the moon. But when the moon kingdom was destroyed, he was sent to Earth with the other sailors, and he lost his memory.
On Earth, Mamoru is a college student who mostly keeps to himself. His parents died in a car accident when he was very young, and he had a hard time remembering his past. He often had strange dreams about protecting Princess Serenity. They made no sense, because he didn't know who that was! ^_^ But eventually, he found that he was Tuxedo Mask, the sworn protector of the Sailor Scouts, especially Sailor Moon.
Tuxedo Mask always comes in just the nick of time to help or save one of the sailors. He throws a single red rose which stuns the enemy. He then reprimands the monsters, and gives the sailors a quick pep talk, always reminding Usagi to use her power to eliminate the monster. In the manga, Tuxedo Mask had some other powers, including telepathy, and energy blasts from his cane, which he could also use to deflect attacks.
Mamoru and Usagi are destined for each other. Usagi hated Mamoru when she didn't know he was Tuxedo Mask, but had a crush on Tuxedo Mask. I (Rei) was going out with Mamoru when they found out each other's identities, and their true love was revealed. Of course, being as sweet as I am, ^_^ we broke up so he and Usagi could be together. I was sad, but I did it willingly and never said anything. Mamoru affectionately calls Usagi meatball head, and they get along well most of the time.
But then, Mamoru had dreams of Usagi being hurt while around him. He broke up with her to protect her, and broke her heart. She was crushed, and we all had no idea why he had done that. When he found out the Dead Moon Circus had been causing the dreams, the two quickly got back together. Since then, it's been (mostly) smooth sailing between them.
Geocities