The King's Beast. Volume 4: 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!
Someone in the imperial palace is trying to kill Prince Tenyou, and Rangetsu has reason to believe it might be the icy third Prince, Kougai. When things turn violent on the jiju field, Rangetsu decides to confront Prince Kougai directly-and ends up as the prize in a wager between royal brothers!