Darien/ Tuxedo Mask
Darien is Tuxedo Mask. Tuxedo Mask showed up mysteriously in the first episode and the scouts
didn't find out who he was until the episode where the Silver Crystal appears. Till then, he is seen
often teasing Serena about her hair (sa: calling her "Meatballhead") or otherwise annoying her.
Darien is Prince Darien, from Earth, as well as Tuxedo Mask. He and Princess Serena were to be married in the Moon Kingdom, but when Queen Beryl and the Negaverse came, it kinda wrecked their plans.
Tuxedo Mask was created by Prince Darien so that he could protect Princess Serena. When Darien was sent to Earth and lost his memory (with the others) his subconcious still made him become Tuxedo Mask and save Sailor Moon (Princess Serena) whenever she was in danger.