From Bitter Root Wiki

Poisoned by greed, hate, racism and several other evils the Jinoo are former humans who have been warped into monsters by their hatred. Previously human, the Jinoo's bodies have been warped to match their personalities on the inside, and are no longer evil. The Jinoo are the main antagonists of the Bitter Root series and find themselves the enemy of the Sangerye family.

Despite their evil nature, the Jinoo find themselves thrust back into the darkness by the Sangerye family — who have been fighting the Jinoo for generations — and their Fiif'no Serum, a mysterious creation capable of returning a Jinoo back to their human form.