5-nin no Ou by Eniwa

Plot:
The kingdom of Jebron is ruled by the Five Kings, each born from the blood of a god. Sage, a young orphan, has only one purpose—to protect his little sister, Hoshimi, who possesses divine powers. To ensure her safety, he offers himself to the cruel and arrogant Blue King, placing his life in danger. However, fate leads him to an unexpected encounter with Gules, the compassionate and kind-hearted Red King. As Sage finds himself drawn to Gules, emotions begin to stir—but the Blue King, who claims ownership over Sage, sees this as nothing less than an insult.