The King's Beast. Volume 3: Rei Toma

A smoldering tale of romance and revenge set in the world of the New York Times best seller Dawn of the Arcana!
Ajin boys who show signs of special abilities are conscripted to serve in the imperial palace as beast-servants-status symbols and shields for their royal masters, to be kept or discarded on a whim. When they were children, Rangetsu's twin brother Sogetsu was ripped from her arms and sent to the palace to attend Prince Tenyou as a beast-servant, where he quickly fell victim to bloody dynastic intrigues. Now in a world that promises only bitterness, Rangetsu's one hope at avenging her brother is to disguise herself as a man and find a way into the palace!
Rangetsu hasn't given up on vengeance, but nothing about the imperial palace is simple. Now that she knows and trusts Prince Tenyou, she has come to see that her most important duty is to keep him alive for the sake of all Ajin-even at the cost of her own life!