Defeated and condemned from the planet she attempted to conquer last, Ultimecia used her powers that allowed her to travel through space, escaping her certain death. Ultimecia was a sorceress of white hair and purple marks on her face, her powers were god-like, she did not belong among mortal men, but her arrogance was her biggest flaw and the reason why she made a grave mistake. The spite towards those who have done her harm has grown into cold bitterness, the same reason why she wants to avenge the death of her loved one. That bitterness was the main result of the violence she has caused. She left death and ruins behind her, people gathered to stop her, but it was impossible to strip Ultimecia of her power - they needed to destroy her instead.
Of course, the sorceress would never allow anyone to take her alive, so she decided to find herself a new home, with a great and dangerous ambition. However, the journey through space did not go well. Using so much power at once without rest, while being wounded, made her lose control and she fell into the atmosphere of a specific planet: Gaia.
The landing was hard and she left an enormous mark in the ground where she fell. It created what is now known as the North Crater. She survived the fall, but the living conditions on this planet was wasting her body away. The fall did break her one wing, her other black feathered appendage was functional, but she couldn't fly away in the state that she was. As she wandered the unknown world, with struggle and ticking time, she did run into Cetra, the city where the Ancient lived. Ultimecia was not welcome there and in agonizing pain, she used what was left of her powers and destroyed Cetra, along with the people within it. Those who survived were tainted by her otherworldly magic and they turned into monsters. It turned out that her blood, which was turning black when she crashed into Gaia, affected the people here rather... Differently. There was one that she kept alive and Ultimecia left her own body, only to possess the woman she spared, the woman who was named Edea... The body that she still to this day owns. Her old body crystallized itself before it completely decayed, considering that it was now nothing but a lifeless corpse, with valuable genes...
Edea afterwards fell into a coma, a stagnant state, which strangely enough only made her stronger. Edea unknowingly consumed planet's lifestream in very small amounts, it fed her magical powers. Edea's appearance became very powerful. While she did appear human, there were some changes in her face and her aura radiated with Only for her and her body to be discovered two thousand years later by Professor Gast, who claimed that this discovery was revolutionary. The crystallized corpse that was tested and researched showed unbelievable and even frightening results that proved that there might've been some superior beings in the world or even outside of Gaia itself. But the living and stagnant body they found, once they took it to ShinRa building, the woman woke up. They didn't get to touch her, thanks to the manipulative nature, but also her new powers, Edea took over the show. She even supported the experimenting with the body, Ultimecia's unresponsive, yet animated body, which is now known as JENOVA.
Edea is not leaving the ShinRa's Company building, given that her appearance and aura clearly indicate that she is NOT human, but she doesn't need to. She manipulated Shinra, man himself, to work on many scientific projects that will "benefit Midgar and their financial situation". Shinra knew this will bring him more power and stronger reputation. Even better, Edea didn't even want the attention. She just wanted the lab for herself and that's what she got. She has the main role along with Hojo in testing how JENOVA can benefit their newest Jenova Project, more commonly known as the SOLDIER.
Trained men who were infused with Mako, disciplined and loyal men who protected the people. That's how they were presented to the public. Edea, however, had another idea. She went for a subtle approach, unlike the last mistake she has made and instead worked her way slowly in creating brainwashed minions. But she couldn't have small and puny minions. She wanted them to be strong. While Mako injected SOLDIER were impressive with their progress, it's when she had a bigger idea - infuse them with JENOVA cells. And little did she know that she has hit the jackpot...
This is how she created Angeal, Genesis and lastly, Sephiroth. The one winged angels, wings that matched Ultimecia's. Nobody met her firsthand, nor did anyone from the SOLDIER knew about her, but she knew all about them. It wasn't long before she took interest in Genesis, who romanticized the idea of some Goddess from a silly poem. She wanted to play around and introduced herself to Genesis, only for him to hopelessly fall in love and address her as "Godddes" whenever he sees her. He would obey her call and every order and kept her secret safe with him. Genesis became Edea's eyes and ears all over Gaia, did every little thing he was asked to do. But it didn't take long before all of the given orders were mostly focused on the First Class SOLDIER, his friend, but also his rival. He noticed that Edea's interest in him was fading and was focused on the silver haired man. This did not bode well with Genesis and he has felt the envy and jealousy to gnaw at him from within.
However, it wasn't long before Edea's attention was completely turned to Sephiroth and she only increased the cells from JENOVA. He even eerily resembled Ultimecia herself, as it turns out, he's the only one who had a wing that came from his right, while Jenova had a wing on her left, much like Angeal and Genesis did. Edea began to think of Sephiroth as her other half and secretly began to wish to meet and talk with him personally, not to hide from him or anyone else in the world.
Her further experiments were focused on wiping out the excess population on Gaia, making all of the SOLDIER mistakes into monsters. She wanted to be rid of everyone who would consume Mako and lifestream in general, which would further limit her power and ruin her plans. That also means, that the large mastermind will not be able to remain within the ShinRa building forever.