Tensei Shite mo Kiraware Ouji Datta no de Kankei Shuufuku Ganbarimasu.

Reincarnated from a lackluster office worker into Ashel, a prince in another world, he initially expects a "cheat life" with his angelic appearance. But soon, through others' reactions and resurfacing memories, he recalls that the previous Ashel had a terrifying personality. Feeling isolated and lonely, Ashel particularly notices the cold demeanor of the black knight, Theodore, who seems to hold special feelings toward him—but what could they be...?

(Source: Kadokawa, translated)