απρόθυμα

tl;dr - Sheba does not fall to Weyard at all, but is instead raised on Anemos to prepare her for a ritual she must perform in the Anemos Inner Sanctum on her fifteenth birthday. However, she is reluctant to perform this ritual, as performing it means certain death. She descends to Weyard a year before the ritual must be performed and joins Felix's group in the hope that traveling will help her to find another way.

THE RITUAL
Thousands of years ago, to avoid a great calamity, the city of Anemos was raised high into the sky where it could be free from the poisonous influence of Alchemy on Weyard. This feat was made possible by the fact that Anemos was a city of Jupiter Adepts, though at the time they were called (and still are called, as they have been untouched by the changes in Weyard) Jenei. Because they were born with an inherent control over wind and lightning, the five strongest Jenei of Anemos were able to perform a ritual in the Anemos Inner Sanctum in nearby Contigo to raise the city.

The price of this feat? Their life energy, saturated as it was with the influence of Jupiter.

Unfortunately, life energy is not everlasting. Their collective life force sustained the city for nearly five hundred years before the elders noticed that something was wrong. Examining the records of the event identified the problem, and a descendant of the ancient hero Yegelos was dispatched to the surface to perform the ritual in the Sanctum. And so it was that the tradition of renewing the spell once every hundred years fell to the descendants of Yegelos.

BIRTH AND CHILDHOOD
Sheba was born into the bloodline of Yegelos, a few months after the birth of a male cousin, Lysandros, on her mother's side and before the birth of a female cousin, Elysia, on her father's side. As the three children were born in the proper time period for a renewal of the spell keeping Anemos afloat, they were collectively taken to a temple when Elysia was one year old, and their futures were read. Sheba was identified as the descendant of Yegelos that would go to the Anemos Inner Sanctum and give up her life to keep Anemos afloat.

Though she was marked for death, this was considered a great honor in Anemos. However, though she had the respect of many from a very early age, there were quite a few things she wasn't allowed to do lest something happen that would interfere with her performing the ritual (because she had to die at the age of fifteen and not a moment sooner). She was kept with her family until she began walking and talking, at which point she was given over to the care of priests and priestesses in the temple.

The reasons for this were twofold: Firstly, to prevent her family from getting attached and trying to spare her from her fate (as had happened with the last ritual a hundred years before), and secondly, to commence with nurturing the Psynergy within her. The more power she possessed prior to the sacrifice the more effective the ritual would be. A heavy emphasis was put on the development of her combat skills, but not so much on the development of prophecy and mind reading, the reasoning behind this being that it would be best for her not to see a future she wouldn't be a part of and for her not to share the thoughts of people she would never see again after the ritual. Both powers do come naturally to most people possessing the power of Jupiter, though, so she can still do this, merely on a lessened level.

For her part, Sheba understood what was expected of her... but she didn't much like it. Because she was raised with no other purpose, though, she did not argue. It was the fate of members of her family after all.

DESCENT TO WEYARD
Sheba's training continued and would have continued until a week before her fifteenth birthday, except for an incident that occurred when she was thirteen. In spite of the way the development of her prophetic abilities had been left untouched, she was overcome by a vision of herself fighting alongside a brown-haired man, an auburn-haired woman, and a blue-haired man under the beacon of Jupiter Lighthouse. This confused her greatly, as she had believed that in spite of her reluctance she would go to the Anemos Inner Sanctum and complete the ritual at any cost.

Given time to mull it over, she started trying to induce another vision. Shortly before her fourteenth birthday she succeeded and realized that there was a group of people wielding Psynergy (now called Adepts on the surface, though she thought of them merely as descendants of the Jenei) who were attempting to restore the power of Alchemy to Weyard. While she didn't quite agree with the methods of the Proxians, she recognized Felix and Jenna from her first vision, and it was then that she started to form an idea.

"If I traveled with these people who are working to change the world, would it be possible to change my fate?"

After all, the power of Alchemy was said to be immense. If it were possible to restore Alchemy to Weyard, or even if only the beacon of Jupiter Lighthouse was lit, there might be enough ambient Jupiter energy to make the ritual succeed without the loss of life. And if that didn't work... she would have to complete the ritual anyway, but at least she would have tried to save her own life first.

Her mind made up, she gathered her things from the temple in the dead of night and went to the edge of the floating city. Without looking back, she leaped, confident in her ability to control her fall.

VENUS LIGHTHOUSE
The advanced development of her powers (as she had been learning to use Psynery since she could walk) allowed her to control her descent. She landed in the ruins north of Lalivero, but without the crash as she had been controlling the wind currents from the beginning of her fall (as opposed to the sudden use of Psynergy to cushion her fall that she would have displayed as an infant).

The timing was good: the party led by Saturos and Menardi arrived in the ruins within the span of a few hours, amid much shrieking and mourning from the people of Lalivero (as, without a hostage of any sort, they had been forced to burn their way through). Sheba convinced them to allow her to accompany them to the top by opening the way for them and by demonstrating her abilities as what they called a Jupiter Adept, and she made use of her powers (some of which, like her mind reading for the statue puzzle, she hadn't made much use of in the past) to get them through Venus Lighthouse and to the aerie.

The use of her mind reading to solve the statue puzzle gave Saturos and Menardi the idea to have her linger on the aerie to see if Ivan truly bore the Shaman's Rod or not, and so she did. Isaac's party, mistaking Sheba for a hostage rather than a willing participant, tried to negotiate for her safety (and for Jenna's and Kraden's safety) by giving up the rod, but when Felix and Sheba started to leave they realized that they were going to the ship, not to send her back to wherever it was she came from, and the fight was initiated before they could leave the aerie.

The resulting battle ended in Saturos and Menardi being defeated, transforming into a two-headed dragon, and being defeated again. When an earthquake struck the lighthouse, Sheba fell over the side but her hand was caught by Felix, who in a stunning display of heroism (read: rashness), jumped after her when she couldn't hold on any longer. Isaac's party was left stunned. Sheba, meanwhile, was left wondering at the impulsiveness of a Venus Adept trying to save someone who had control over the wind.

THE GREAT EASTERN SEA
It was lucky that Felix jumped after her, because Sheba, having been raised in a floating city, couldn't swim. He managed to bring them to Idejima before passing out, and the events of this part of their journey played out mostly as they did originally - merely without the references to Lalivero, and with a little more initial suspicion on Jenna's part.

Though there was only a year before she needed to be in Contigo to complete the ritual, Sheba welcomed their detours in the Great Eastern Sea as a chance to investigate other rituals. She took a particular interest in Garoh and Air's Rock, even breaking away some of the Psynergy-infused stone of the area to take with her in case it kept its potency by the time she had to go to Contigo, and also in Lemuria when they had the chance to go. When Piers left to do his own thing there, she also left to do her own thing: namely investigate how these ancient people had sustained their land for so long.

Once they had gained the Grindstone, they proceeded to the Great Western Sea. At this point Sheba knew she was going to have to make a decision sooner rather than later, and her uneasiness and reluctance to give up her life was growing, because she was starting to grow really fond of the people she called her friends. She hadn't had any real friends before descending to the surface, you see.

It's a lot to give up, after you've gone without it for so long.

JUPITER LIGHTHOUSE
The events of the Great Western Sea proceeded mostly as normal. Sheba, not wanting to tell her friends everything, hadn't told them that she was from Anemos and could already use Hover, so they proceeded on to Shaman Village as Saturos and Menardi had instilled it in Felix that it would be necessary to reach the top of Jupiter Lighthouse. The encounter with Moapa, where Sheba insisted that even a girl could be a hero, planted the seeds for a slight breakdown as she was reminded of her duty as a hero of Anemos. When she opened Jupiter Lighthouse for Felix, she didn't immediately follow them in, but Jenna wasn't able to convince her to come clean immediately.

They solved the lighthouse's puzzles, watched Karst and Agatio ensnare Isaac's party in their trap, and were tasked with the job of saving Mia by Alex. They also negotiated to spare Isaac's life in exchange for giving up the Mars Star and the lighting of the Jupiter beacon. After a spirited defense of Felix on the aerie, Karst and Agatio left, and Sheba (who had been more and more agitated after the lighting of the beacon) was finally pushed into telling her friends what was wrong.

She told them everything: What her duty was to Anemos, what she had been hoping for when the beacon was lit, and how she was running out of time. Her birthday was in the next two months, and if she went north to Prox, she wasn't sure she could return to Contigo to perform the ritual in time should restoring Alchemy to the world not be enough to keep Anemos floating.

Before they could figure out a solution, though, Isaac made it to the top of the aerie and demanded that they meet in Contigo and talk. They agreed, and he and his party left first. Felix's party followed them down, with Sheba deep in thought about the ritual and what would happen if she abandoned it. The two parties talked and it was agreed that they would join forces and venture north to Mars Lighthouse, though Sheba never spoke up to commit to it. Hamma also said she does not believe the return of Alchemy would provide "all the answers," looking directly at Sheba as she said it, and Sheba took this to mean that Hamma doesn't think Alchemy will be enough to save Anemos. They stayed the night at the inn, and Jenna and Sheba shared a room. They had one exchange before going to bed:

"What are you going to do?"
"...I don't know."

And it is at this point, after falling asleep, that Sheba appears at the Tower of Animus.

REST OF THE AU
If left interrupted, Sheba chooses to go north to Prox along with her friends, clinging to the hope that even if the timing would be cutting it a little close, restoring Alchemy would be enough to save Anemos from falling. However, once there, it is confirmed during their encounter with the Wise One that it wouldn't, and Sheba falls into a sort of despair and grim determination. After they collect the parents that had been left in Prox as hostages, Piers reluctantly brings Sheba back to Contigo before the rest of them go to Vale, and she completes the ritual.

...almost a year later, after Felix leaves his homeland, a girl greatly resembling Sheba appears in Kalay. She is very physically weak and can't use Psynergy at all, but when Jenna calls out "Sheba!", she smiles and calls back to her.