Ceres hails from the Second reflection of the Source, a Sapiana (hyur) citizen of the oligarchy Empyrca (Ul'Dah) from a well-off family. One day, she returned home to discover her parents had engaged her to an older man while she wasn't present to defend herself, as their riches had been drained due to her father's gambling problems and her suitor promised to refill their coffers in exchange for a young bride.
Ceres refused and fled her home, only to realize the futility of it all as she wandered the streets of her city without anything to her name. She considered ending her life in protest of it all, not wanting to wallow in poverty or others' pity, but was suddenly approached a masked man introducing himself as Nabriales. He enticed her with the promise of never feeling helpless again, and upon her agreement, awoke the memories of her past life as the Ancient called Hera.
Hera had been an Ancient who lived in one of the many cities scattered across the Unsundered World, far from yet still obeying the laws of the Convocation of the Fourteen. She contributed to the world in her public service position, ensuring that communications to and from others went undisturbed. Ultimately, though, she lived a rather easygoing life in a beautiful utopia -- and, therefore, strongly supported the Convocation's desires to restore it to what it once was. And, upon seeing what she could have as opposed to the nothing she was now left with, how could Ceres do anything but join the Ascian's ranks?
Unfortunately, it turned out that she was made to be the lowest rank of Ascians: she had her past memories, the knowledge of the Ascians' grander scheme, and their nigh limitless powers -- but she was nowhere near as strong as anyone above her. She couldn't even get stronger if she tried; as someone who had not been part of the Convocation, there was no way for her to ever be more than what she was.
Also, Nabriales was a obnoxious prick whom she quickly came to despise, but had no choice but to obey him lest she be unmade. She did is bidding, manipulated others to help push the Second's chosen hero into dangerous situations and tilt the balance of the elements -- all the while being far more careful not to actually face the infamous Warrior Of Light.
And when Nabriales achieved his mission, tipping the scales so that endless fires ravaged the world, Ceres and her fellows were summoned back to the Void to await her next orders. Her soul was torn from the body she had been born into unexpectedly, and although new bodies could be made into her preferred image, it was still a horrible experience.
Nabriales did not care enough to keep track of the minor Ascian lackeys who did his bidding, and the Unsundered only made note of the collapse of the Second as a whole on the path to Zodiark's restoration. When Nabriales was defeated, Ceres was sent to the Source, only to meet her end at the hands of the Shadowhunter, Gaius van Baelsar.
Of course, it wasn't a permanent death -- she returned to the Void, frustrated with being treated as canon fodder. Before she could be sent on another mission, however, the Warrior of Light triumphed on the First and defeated the remaining Unsundered. With them out of the picture, the Ascians were in disarray -- and Fandaniel was far more interested in his Telophoroi cult than organizing the leftovers.
Ceres took the opportunity to take on the life of a noblewoman in Ul'Dah, which reminded her of the place she had been raised in as a girl. She purposely dropped off the radar, and intended to do so forever, until Zodiark was destroyed during the events of Endwalker. As powerless as she was, Ceres could feel something calling to her.
She ended up on the moon's surface and lingering over the chasm that contained his remains. Ceres was able to absorb a fraction of Zodiark's power -- perhaps a tenth of it at most. This didn't elevate her to the status of an Unsundered, but it did strengthen her enough to equate to someone roughly of Fandaniel's ability.
Pleased with this new revelation, Ceres returned to her home to contemplate what this meant. But the Warrior of Light dispelled that piece of Zodiark before she could take a second hit.
Ceres is trying to continue to stay under the radar and not draw attention to herself, but now that she's absorbed some of Zodiark's remains, there's a gnawing desire in the back of her mind to find more. Maybe a few trips to other reflections' moons are in order...?