Mamoru Chiba (地場 衛 Chiba Mamoru) 
A student somewhat older than Usagi, called Darien in the English adaptations of the series. As a young child he was in a terrible car accident that robbed him of his parents and his knowledge of who he is. During the series he has some precognitive ability, including dreams that inspire him to take on the guise of Tuxedo Mask and fight alongside the Sailor Senshi. After an initially confrontational relationship, he and Usagi remember their past lives together and fall in love again.
Tuxedo Mask/Tuxedo Kamen, as depicted in the manga.
/in anime
Tuxedo Mask is a disguise Mamoru created for himself in order to hunt for the Silver Crystal incognito and, later, assist the Sailor Senshi in battle. In addition to his white mask, he wears a black tuxedo with a black cape (lined red), and in the anime and live-action series carries a black cane which he uses like a sword.
Prince Endymion
Endymion (エンディミオン Endimion, called Prince Darien in the English-language anime) was the Crown Prince of planet Earth during the Silver Millennium. He had four guardians, the Shitennou, who protected him just as the Guardian Senshi did Serenity, the Princess of the Moon. He and Princess Serenity fell in love, and often disobeyed their parents when it came to seeing one another. Partly because of this, the Dark Kingdom was able to create war between the Earth and Moon; the Shitennou defected to help the Dark Kingdom, but Endymion fought for peace and swore to protect Princess Serenity. In the manga, Beryl killed Endymion and upon witnessing this, Princess Serenity killed herself in sorrow. In the anime, Queen Metaria killed them both. Prince Endymion is reincarnated in the 20th century as Mamoru Chiba.
King Endymion
in anime
in manga
Later in the series, Mamoru is shown as King Endymion, King of 30th century Crystal Tokyo. He is the husband of Neo-Queen Serenity, and father of Chibiusa. As king of Crystal Tokyo his royal garments are a white and lavender tuxedo, as well as a royal staff. When first introduced in the manga, he was a computer mirage of himself.
Moonlight Knight
The Moonlight Knight (月影の騎士 tsukikage no naito, literally "knight of moon shadow") appears only in the anime for the first half of Sailor Moon R, the Makaiju arc. When Usagi wished to be a normal girl at the end of the first series, everyone's memories were hidden, including Mamoru's. However, Mamoru subconsciously still wanted to protect Usagi, so his memories separated from him and concentrated into the Moonlight Knight. The Moonlight Knight could co-exist in the same space and time as Mamoru. Like with Tuxedo Mask, Usagi thought every cute guy she knew was him, and was surprised when she discovered who he was. Once Mamoru recovered his memories, the Moonlight Knight disappeared and rejoined with Mamoru. Mamoru returned to being Tuxedo Mask after that point. The Moonlight Knight mainly dresses in Arabian robes and carries a saber. He also throws white roses, instead of Tuxedo Mask's trademark red roses.