Aijin are beast-like beings that, despite their extraordinary abilities, are subjugated by humankind due to their smaller population. While male Aijin are required to serve in the military, their female counterparts are forced to work lowly jobs.

Born as Aijin, Rangetsu Ko and her younger twin brother, Sougetsu, fend for themselves. Rangetsu is soon devastated when Sougetsu's powers are awakened and he is taken away to become the prince Tenyou's beast-servant.. Not long after, she hears rumors that her brother has been killed by Prince Tenyou himself. Brimming with anger, Rangetsu disguises herself as a man and enlists in the military, eventually becoming Prince Tenyou's next beast-servant.

Although her opportunity to exact her revenge is within reach, Prince Tenyou defies Rangetsu's expectations, and she begins questioning the circumstances surrounding Sougetsu's death. Pursuing the truth leads Rangetsu into a political game far more dangerous than she could imagine.

Included one-shot: Volume 1: Suijin no Hanayome Tokubetsu-hen (The Water Dragon's Bride Bonus Chapter)

Ou no Kemono: Enpei no Arcana was nominated for the 68th Shogakukan Manga Award in the shoujo category in 2022.

The series has been published as The King's Beast by VIZ Media under the Shojo Beat imprint since February 2, 2021, and simulpub through VIZ Manga. The manga has also been released in German by Kazé since May 5, 2022; in Indonesian by Elex Media since November 10, 2021; and in Spanish by Distrito Manga since March 30, 2023.