Mamoru became an orphan in his early life when his parents and him got in a tragic car accident killing his parents. Thats when he met Fiole his friend which shows up in the Sailor Moon R movie. He also met Usagi at the hospital when her little brother was born. She gave him a flower which he gave Fiole. That made them even closer friends. Mamoru is in the show from the beginning of Sailor Moon R series. He is always teasing Usagi and laughing when ever she stumbles and falls. They learn that Mamoru is Tuxedo Mask and that he is also prince Darian from earth bacj when the moon kindom existed. So Mamoru and Usagi realize that they were ment for each other.
I am sorry for the confusion in the names. I will try to get the japanese names which i don't have at the moment.