When transformed, he turns into Tuxedo Mask (タキシード仮面 Takishīdo Kamen), a very nice but mysterious hero who has the habit of suddenly turning up when Usagi Tsukino needs him the most, during her battles against the evil forces.
Overview
During Usagi's previous life, he was known and eventually remembered as Prince Endymion, the husband of Princess Serenity. While wearing a mask to keep his identity hidden, he usually appears during enemy operations to help the Senshi finish their battle, and sometimes to physically aid them during battle. His first meeting with Usagi in the first season of the anime has a rather typical romance looking scenario around itself. They appear to not be fond of each other at first, but eventually they end up as a couple.
As many of the other characters in Sailor Moon, Mamoru has different aliases based on where Sailor Moon aired. When transformed, he's commonly known as Tuxedo Mask in the English versions, even though the more accurate name, Tuxedo Kamen, remains in the Japanese versions. His civilian name, aside from 'Mamoru' and 'Mamo-chan', is also known to be 'Darien' or 'Dar', even though this happens inconsistently.