Otoshigo to Juusha no Monogatari

Monstrous-looking slave Kustar was waiting for his death in his cage when the aloof but beautiful Julius bought him as his bodyguard. As the second of son from a concubine, Julius is embroiled in various feuds with his family and often have to sleep with male "guests." His cold demeanor towards the pure and devoted Kustar slowly changed as he learned to open his tightly closed heart.

(Source: AlphaPolis, translated)