A Hexer is a Fallen in Oninaki.
Elusive Fallen that can warp across the battlefield. They chant strange incantations that enhance nearby Fallen, temporarily raising the Defense of those affected.
A Daemon's Testimony 3-1
Hex have existed since time immemorial. Fallen arise from negative emotions and therefore sometimes mutate, but these have always existed as they are. Likely they spawn from a basic emotion. Perhaps that explains their strength.