Tensei Idol to Doruota no Kishi

Takao was a knight in a previous life. Now, after many reincarnations, he works as a successful corporate consultant in modern-day Japan. When he catches a glimpse on screen of Hikaru, a member of the boy band Dumas, the memories come flooding back. Hikaru is the reincarnation of the prince Takao had pledged himself to, and whom he was unable to protect. As if to atone for his failure, he vows to serve Hikaru in his present life. How? Well, as Dumas's manager of course. That way, he can watch over Hikaru closely and ensure his future is bright. But when the "untouchably cool" idol singer offers him a rare smile, Takao realizes that the feelings he bears for him (and his former prince)...may extend beyond fealty and duty...!
(Source: MangaPlaza)
Note: Includes an extra chapter.