Reiyon is an Hanjuujin.

Quote from Chelsea when she explains what an Hanjuujin is:

"Long ago, there was some research that inserted animal genes into humans.

What was born from that are the Hanjuujins.

The Hanjuujins are equal in strength to the elemental uesrs.

But for some reason... The research was suddenly interupted.

And the Hanjuujins were sent elsewhere."

Reiyon first appears in the end of episode 21, but isn't introduced untill later in episode 22.