He is actually Mamoru Chiba, the reincarnation of Prince Endymion, who is deeply in love with Usagi (Sailor Moon). He chooses to use the identity of Tuxedo Mask to fight alongside Sailor Moon and protect her, even if it means putting himself in danger.
He never experiences any loss of memory or change in personality due to brainwashing. There might be some confusion with other characters or a different storyline you're thinking of. Could you please clarify what you're referring to?