Tuxedo Kamen is the masked man that usually comes to help Sailormoon in time of need. In the manga Mamoru is around 17 or 18 years old. In the anime he is older. Chiba Mamoru's parents were killed in a car crash on his 6th birthday. Mamoru miraculously survived, but he lost all of his memories. He thought the only way to get his memories back was to find the Ginzuishou becaus he had been having dreams and this girl would keep saying "Maboroshii no Ginzuishou". He kept trying to find out who she was, but he would wake up before her identity was revealed. When he first met Usagi, all they did was argue and yell at each other ^_^; But when they found out each other's identities they recalled that they had been lovers in their previous life. Mamoru was actually Prince Endymion from the Golden Kingdom on earth. In the manga it is revealed that Mamoru carries the Golden Crystal. As Endymion, he had four guardians, Kunzite, Zoisite (who were not gay in the manga) Nephrite, and Jadeite who fell under the evil control of Queen Beryl. In the first series, Mamoru is brainwashed by the Dark Kingdom because Beryl was infatuated with him. Beryl had always been jealous of Princess Serenity because Endymion chose her over Beryl. He fought against the Inner Senshi and Sailormoon at Beryl's command. At the very end of the series, Sailormoon managed to refresh his memories of who he was but then he was killed by a shattered crystal thrown by the wicked wicked Queen Beryl.
In the R series, Mamoru was slow in getting his memories back. The inner senshi were all revived by Luna. So the Tsukikage no Knight (who was not in the manga) takes over his job of protecting Usagi. The Knight was actually Mamoru's feelings to protect Tsukino Usagi. At the end of the Makaiju series, the Knight returns to Mamoru's body. In the second part of the R series, Mamoru dumped Usagi (it was so sad!!) because he had a bad dream that said that she would be hurt if they stayed together. (Did not happen in the manga) They eventually hook back up and Mamoru is revealed to be King Endymion, Chibiusa's father.
In the Sailor Star series, Mamoru boarded a plane to America because he was going to study at Harvard. Usagi never heard from him and she worried about him, but she didn't tell the other senshi. At the end, 
it was shown that Galaxia had killed Tuxedo Kamen, when his plane was still in flight. But, like everyone else, he is revived at the end and led back to Usagi by ChibiChibi. ^_^
In the anime, Tuxedo Kamen wasn't really given an attack except for his Rose Throw and his cane. In the manga, he had the attack Tuxedo La Smoking Bomber. Tuxedo Kamen has been involved in powering up Sailormoon several times. Like in the Super series when Sailormoon got the Spiral Moon Rod. Mamoru and Diana (Chibiusa's kitten) are very good friends. ^_^
My Opinion:A lot of people say that Mamoru is not the best boyfriend for Usagi, but I think he is. He loves her very much and would give his life to protect her. He isn't really that bad.