(TLDR at the bottom if you need it)
Long before Onnen became a holiday destination for the people of Canelux and Parvpora, there lived an elder sage in the east. He came from the old continent when Varya was still alive and at a time when insanity had yet to grip the eastern pantheon. It was a relatively peaceful era, one where power was far easier to come by and where the gods had yet to suffer greater controls on their power.
Yet just the same as it always has been throughout time immemorial, the presence of peace breeds contempt. So long as the social ladders remain concrete and the movement among their steps remains stagnant, rebellion is inevitable. And so we come to the story of that sage in Parvpora: a bloodmage of already incredible ability who had been exiled from his homeland for trying to amass enormous power without a mind for the consequences.
His name is no longer important to this tale, for it was erased the moment he crossed the line that made Lucina. He came to Parvpora directly off of his exile from Onnen, and with the intent to usurp Varya's power for himself: to use it for his own devices and goals. But great power requires a sibling to attain it. The nameless sage knew this lesson well, and realized that, despite the sum of all his current assets, he did not yet possess the strength to bend the tree to his will. Instead, he would have to start small, and so he began stirring one of the earliest cults to ever exist in Abed: one composed entirely of people allying themselves against the eastern pantheon.
That bloodmage cult grew over a number of years, sometimes by blackmail, other times by manipulation or force. Eventually, though, it was large enough for the founder to make his first move.
To bind a djinn, even back then, was considered a dangerous exercise. Near limitless power and with incredible rarity, these creatures were thought equal or even superior to the gods themselves. The elder sage knew he needed that kind of power in order to subjugate something as powerful as a world tree, so he set about on an insane quest with his followers in tow. He devised a new blood ritual based on theories he had held for many years, bringing together many sacrifices in order to lure a young djinn into a cage in order to seal it inside. His lackeys were designated collectors for this plan, and so one of the largest blood magic rituals that no one has ever heard of was set into motion.
They took a mermaid, a vampire and several slaves first. The slaves they tortured until just before death, tenderizing them with the anguish that their founder's ritual required and then reviving them so they didn't die before their time. The mermaid and vampire they locked up for the final sacrifice, gagging them and feeding them just enough to sustain themselves - sometimes even to the point of forcing them to eat. Meanwhile, the lackeys of the cult also went out to fetch 12 sacred animals that had influences from the tree as well as a drop of blood from the high dragon who still existed in Baltil at the time. Everything was collected before the first day of spring, and so the ritual began on time.
The blood of the sacred animals now slain was mixed with that of the dragon on the day of the ceremony, and the vampire as well as the 700 or so slaves that had survived shared in their fate. The mermaid, not destined to share their fate, was the only one left alive to witness the carnage. She was forced to scream out in horror as magic filled the room, bringing the target the djinn to the waiting cult and, more specifically, to her. She was made into the vessel to contain the djinn and the souls of everyone that had died, molded into something different from all the other races.
In the end, the elder sage and his fanatics succeeded. Their profane ritual had created a little girl from the body of the mermaid, Lucina; and it was she who contained the essence of everything they had dreamed for, she who was to become their greatest weapon.
Yet, as is typical for most mortals who grow far too prideful for their own good, the cult and its master suffered a swift but spectacular fall in their moment of triumph. The deities of the east, incensed by the theft and eventual murder of their treasured companions - not to mention the abhorrent sacrifices that had been performed in their sacred abode - lashed out in the immediate aftermath of Lucina's birth. To punish the sage who thought so highly of himself and had orchestrated such atrocities, they placed a curse on his creation. The newly bound djinn was to reflect the evil in the hearts and minds of any person she gazed upon; and so, as soon as her eyes fell upon the murderers who would call themselves her parents, they were already dead. The little girl who had yet to even receive a proper name involuntarily slaughtered the people who had created her at the whim of those curse laden eyes, and so the Mirror of Sin was born.
Unfortunately, the deities released something they couldn't control with that action. Their curse was meant to be temporary, but it got tangled together with the blood magic of their fallen foe as well as the hatred of the sacrifices he had used to create such a blasphemous creature. The original curse had mutated in such a way that it became resistant to the divine attempts to cure or purge it, and their prodding in an attempt to cure just increased that resistance over time. Worse yet, the Mirror of Sin reflected their powers back at them, proving to be dangerous to confront for mortal and immortal alike. While not an official godslayer, she behaved like one, and so the divine were forced to shepherd her away as best they could.
For centuries after that, the Mirror of Sin wandered, never being able to call any one place home. All those who met her during this exile found themselves at the end of a most unfortunate fate, and the deities who originally caused her ailment fell one by one. Eventually, people just forgot about her existence entirely along with the events that led up to it; at least until Natsumi herself had a first encounter.
As a deity of secrets, the Nameless One's self-imposed duty back then was to track down creatures like Lucina: to purge or save them as she saw fit and was able to do. Lucina was one of her top targets in that position, and she was well aware of her existence shortly after rising to power. At first, this awareness transitioned into intent to kill because of how dangerous she knew the Mirror was; but that determination faded when the actual meeting between them took place.
Lucina - as Natsumi found out - had actually learned what she during her exile, and she had taken to hiding her eyes for fear of it. The once small girl had grown into a young maiden, and now lay as a pathetic mess when the white fox found her. She was nowhere near the monster that Natsumi had built her into, and so the goddess relented. Drawing down from her blood lust, she decided to undertake the task that others of her kind had failed in before: to find a cure.
Natsumi codenamed the girl "Anomaly 10666: Mirror of Sin" after that meeting, and proceeded a long, drawn out process of trying to unravel the ancient curse. She did not, however, actually succeed in even partially breaking it until long after she had resigned from her post as a deity. But, by then, she knew that no divine power - at least none that anyone around her possessed - would have successfully broken the curse anyway. Only a specific set of steps that even a mortal could perform would even have a chance, and it took her a while to find those steps. Lucina had been locked in a vault for her own safety until then, and only after unsealing that musty cage was Natsumi finally able to free her from the living nightmare that had claimed so many.
In return for forcing her to wait so long, the white fox gave Lucina the name she goes by today and officially apologized for taking so long. The Mirror of Sin was freed, and she became the lovable girl known as Lucina who was swiftly adopted into the Yamauchi family following her release.
Unfortunately, however, the curse proved too stubborn to remove entirely in the end. While she is no longer nearly as dangerous as she once was and can live a modestly normal life, Lucina is still susceptible to the alignments and personalities of those around her. She fears what might happen if she looks at the wrong person, and so she conceals her eyes from the world. Because, no matter how far she has come from the bad old days, she still remembers the horror of what it is to be an observer in a prison of your own mind.TLDR:
Lucina was a former patient of Natsumi's - current Nameless One and former deity of keys and secrets - and a former weapon of a cult that has been long dead since before the Parvporan tree fell. The cult used blood magic with a complicated ritual in order to create her, but the ritual angered the eastern pantheon in the process. They laid a curse on Lucina in order to punish the cult and its leader, but, while their methods accomplished their initial intent, they also quickly spiraled out of their control due to unforeseen circumstances. Lucina, as a result, was shepherded into a centuries long exile, and that is where Natsumi found her well into the future.
Though it was delayed by a number of years after their initial meeting, the white fox woman did eventually find a cure for the worst parts of Lucina's curse, and, while it was never fully lifted, the girl still enthusiastically accepted an invitation to join her new family after the fact. Lucy, however, never completely got over her tendency to wear blindfolds. The practice became part of her very existence during her exile as a survival and safety method, and so, even now, she lives in fear of the knowledge that something could still go wrong if she looks at the wrong person with her naked eye.