"You will soon have to prepare for a massive battle, this will determine the fate of your world and you are one of the main factors." ― Xenatra
As per explained by Apollo: Somewhere in history a being once siphoned a finite yet minuscule portion of Omnidea's soul, not being able to contain such power and the curses that came along with it, they banished the soul into a seed which became sentient in its own right. That seed roamed the world in search of a vessel to inhabit and in due time failed. It gave up and lie onto that of the Earth's soil, growing to be a flower of irresistible beauty until one day a wonderous woman found such a plant and decided to harvest it for her garden as a tribute for her husband's lost family. After several years it finally met a human it could take over that was full of youth. It isn't legal by deity standards for humans to possess the souls of Gods or even parts of their souls for that matter, but since she has merged with Amelia completely, Apollo allowed it.
In the beginning Cematra seemed to have a seemingly insane personality as if her mind wasn’t completely there. She acted as if she was crazy and in the beginning, didn’t care for Amelia’s well being too well. After being locked up with Amelia in a cell in the Underworld, the two started to bond a little and Xenatra allowed Amelia 1/10 of her power to help her escape (probably due to not wanting to get removed from Amelia’s body). As of currently, Amelia and Xenatra are closer than they’ve ever been, Xenatra giving Amelia advice and warning her of a large battle that is soon to come between Amelia and Olivia soon.
As most would guess, Xenatra is neither human nor hybrid, but a complete ghost. She exists as part of Omnideas spirit and seems to possess all of the qualities that a ghost would have. It seems that she is somewhat limited to what she can do without a host, due to her not doing much but scare Amelia when in her true form, but allowing Amelia a plethora of new abilities when they’re conjoined.