Saihoshi The Guardian

The spoiled Prince Anel must travel without military escort to a neighboring kingdom where tensions run high. Sastre, an honorable Guardian, is brought in to be Anel's only protection for the journey. Anel is enchanted by his noble bodyguard, but Sastre becomes more interested in Kaleth, the spirited servant and lover of Prince Anel. Kaleth is forced to disguise himself as the prince to be a decoy for his enemies and proceeds to stir up unfamiliar feelings in the heart of the austere Guardian. Sastre finds himself torn between duty and desire.

(Source: Goodreads)