When Mamoru was very young, his parents were killed in a car accident. He had amnesia after the crash. He grew up on his own, apparently, and got a nice apartment. He went to school and studied, basically everything you'd expect a young man to do. Somewhere along the line, he became Tuxedo Kamen, helping the Sailor Senshi out, and found out he was the reincarnation of Prince Endymion of Earth, fiance of Princess Serenity-- and her new form was a girl he didn't exactly like, Usagi. They eventually ignored this and fell in love again. In the future, he's King Endymion, and has a daughter named Chibi-Usa.
In both the anime and the manga, Mamoru is reserved, sometimes sarcastic, and seems to be fairly intelligent. He studies, but isn't really a study freak. He treats Usagi well and acts in her best interests, acting to ensure her future happiness rather than his own.
In the anime, Mamoru did not realize he was becoming Tuxedo Kamen for some time. Huh? That's right, he would turn into Tuxedo Kamen and fight evil, but never know it. He had strange dreams about Princess Serenity at night, telling him to retrieve her ginzuishou for her. He is in college. When he was at the hospital after his parents' death, he met Usagi, who gave him a rose, and also became good friends with a boy named Fiore, who was really an alien, and gave him the rose when he left Earth. Fiore has an attachment to Mamoru even when he's older. Also, he has a form in SMR called Tsukikage no Knight (Moon Shadow Knight, mistranslated by DiC). This is his "spirit" that still wants to protect Usagi after he is reborn. It's pretty stupid...
In the manga, Mamoru knows he's Tuxedo Kamen. He is in high school when the series begins. He seems more mature and even looks more mature.
Mamoru has short black hair and blue eyes, and is tall. In the manga, his left ear is pierced and he sometimes wears a small gold hoop in it. He usually wears semi-formal clothing.
Tuxedo Kamen throws roses to distract enemies in the anime, but has an actual attack in the manga, "Tuxedo La Smoking Bomber". He wears a black tuxedo with a matching tuxedo shirt and vest, plus a white bowtie. He has a charm of sorts in the manga. He wears with this a black cape lined with red, a top hat, black shoes and white gloves, and sometimes appears carrying a long cane. He wears a white mask of some sort over his eyes. He has a crystal of some sort in the manga, similiar to the ginzuishou, but I don't understand what it's for.
I hate him! He gives stupid speeches, he acts stupid, he's just plain stupid, IMHO. I really don't like him. I don't mind him as much in the manga (maybe cuz I can't read what he's saying? ^^;; ). I just don't like him, grrr... But you have to give him some credit, he does like chocolate... :) ... But this is the horrible, horrible man that fathered Chibi-Usa! How could he?! Jerk!
Senshi Name*: Tuxedo
* Naoko Takeuchi has said that he's basically the Senshi for Earth.