Disclaimer: I don't own Legend of Zelda etc
The story summary says it all. This story assumes Breath of the Wild follows the Child Timeline (SS-MC-FS-OoT-MM-TP-FSA-BotW).
While the intial inspiration for this setting is Zelda's dialogue in Memory 1, Nintendo has made it clear that this dialogue is an easter egg, and not a canon confirmation of timeline placement - in German, the line reads something like "whether the hero crosses the sea or forges a link to the distant past".
That being said, enjoy your angst cake.
Chapter One: In the Heavens
One hundred years has messed with his head.
He cannot describe it fully. One hundred years of being awake, but not alive. One hundred years of pure solitude, yet always shared with the cursed Windblight that killed him. One hundred years of being trapped in Vah Medoh, of knowing nothing beyond its mechanical walls, yet somehow witnessing all of Hyrule's cataclysmic downfall, and its tragic, slow demise afterwards. One hundred years of uncertain limbo, and the second the Travel Gate glows to life it takes him a moment to place the person he already just knows showed up.
"Making me wait one hundred years is a bit... indulgent," Revali sneers, because he's not about to let Link know how glad he is to be able to communicate with somebody, anybody, other than the Blight that has howled rage and wrath at him nonstop for a century. Link looks up as though searching for Revali, but he has to know Revali is dead.
Still, the mere boy won't be able to save Hyrule without Medoh's help, and Revali's century-long resentment of Link pales in comparison to the toxic vitriol he feels for Ganon. Windblight would know - they've become well acquainted with their respective hatreds over the last one hundred years. So Revali puts on airs as he guides Link through his Beast with his disembodied voice, knowing all the while that if he still had a body, even an incorporeal one, he would jump at the chance to assist Link, lead him through the hallways, show him all the mechanisms, just to get back at the demon that bested him. Just to make sure the rest of Hyrule doesn't follow him to the grave.
And, from his place in nowhere, he observes. Link seems confused as to where things are - not surprising, given that Revali had never brought Medoh low enough for Link to board, but the Divine Beasts have similar engineering at heart, and years of following Zelda like a puppy should have given him some idea of what to expect. His eyes sometimes glaze over, once he almost tries to walk across a metal beam far too narrow to support him, and he seems bothered by headaches of some sort. After Link eyes some railing across a chasm and fails to find whatever weapon he wanted in his inventory, he pauses and actually takes a seat, looking both frustrated and lost. The hesitation is short-lived, however. Link composes himself and takes a good long pause to compare the Slate's map with his surroundings, then forges his way through the Beast with practiced ease. Revali almost convinces himself he's imagined it.
Then Link seems to have trouble with the guardian scouts and the cursed skulls spit out by the Malice. He lets a shot through his guard that never would have gotten close to him back in the glory days of the Knights. He stops to eat his packed meals more often than the Hylian Champion that Revali remembers - or maybe he's just trying to give Link's gluttony the benefit of the doubt. Yet that former Hylian Champion would wear himself down to the bone and only stop if directly ordered by one of his superiors or the other Champions. He seems weaker. It displeases Revali. It pleases him. It makes him worried.
Why, exactly, had Link taken a hundred years, anyway? From his prison and grave, he'd heard Rito whisperings about the death of all five Champions. He'd never been sure whether or not to believe them. He hadn't wanted to believe them. And Link is here, alive and fighting, so clearly the rumors were wrong. But something must have kept him back for so long. He puts the thought aside to ponder later. Time is of the essence.
As much as he would love to preen over Link's decline in strength, Hyrule has run out of time. Link cannot fail here no matter what. So after Link activates the last terminal, as he makes his way towards the main control unit, Revali speaks again.
"Wait, Link."
Link comes to a stop.
"The demon that bested me - Windblight Ganon - still possesses Medoh. It will not relinquish control without a fight to the death. I can feel it fester with rage that you have almost fully activated the Divine Beast. Once you start the main control unit, it will attack you. Take this time to check your armor, your weapons, your curatives."
Link blinks, nods, and sets to doing just that. Revali inspects his work. He has an impressive handful of bows - nothing compared to his, of course, but worth a tail feather or two - and an alarmingly large hoard of arrows of all kinds - even some tipped with glowing blue blades that scream of ancient Sheikah handiwork. He has powerful blades, even his precious darkness-sealing sword. He's acquired the famed Hylian Shield that the Royal Family bestowed only upon the best of the best, and Revali restrains a huff. Link will need it. His armor is in prime condition, his clothing is suitable for the cold, his stores of elixirs are numerous, and his resolve is unshakeable. As Link eats a hearty truffle on a skewer, Revali describes his own battle with Windblight - the attacks, the rage, and his own bitter defeat.
Hyrule is far more important than his old unreciprocated rivalry, he tells himself as he hates the story he recalls.
At least he can console himself that it's worth it. Link activates the control unit and does not even flinch as Malice erupts from the machine and entwines itself with ancient energy to form the monstrosity that Revali can't keep himself from hissing at from beyond the grave. "Good luck! Remember that it plays dirty," Revali spits, and then he falls silent because the last thing a warrior needs is distraction.
And Link charges with a controlled ferocity that would make any warrior proud. Apparently he'd looted multi-shot bows from several Lynels, and mercilessly pummels Windblight in the face with an ancient arrow volley. In no time at all the demon is in its death throes, and Revali decides that it's better to be relieved and proud rather than bitter and jealous.
But then, Link puts on quite the display. In a maneuver neither Revali nor Windblight expect, Link rides the updrafts and stuns Windblight in place with a well-timed toss of his great thunderblade, and then he's landing on Windblight's back, hacking away at the center while the Master Sword glows with holy light, and suddenly a horrible screaming fills the air as Malice spills and dissipates. Before Revali can really take in what just happened the demon is gone, and Link has collected its purified life force, and the main terminal glows the proper color, and suddenly Revali has a form again.
A spirit form, true, but at least he can manifest. "Well, I'll be plucked..."
Perhaps not so weak after all.
He swallows his pride and accepts Link as a warrior, and because his power over the wind is useless to him now, he concentrates that small portion of his soul and bestows it upon Link to use whenever he needs.
"I must say... that maneuver you did at the end there was a bit much," he quips, and he's surprised to see Link wince. But then the Hero begins to glow golden as Revali sends him back down to the village, and he continues. "I'm going to set Medoh's sights on the castle now. Your job is far from finished, you know. You've kept the Princess waiting for a hundred years."
A hundred years. Of being awake but not alive; of being unaware, yet acutely, painfully so. Of utter solitude; and of the omnipresent oppressive company of his killer. All brought to an end in the space of a day by the Champion who only ever saw him as an ally and not a rival despite Revali's antagonism. The one who outshines them all. Revali turns so he does not see Link depart, so Link does not see whatever emotion warps his face.
But once Medoh takes his perch and focuses his bead on the castle's sanctum, Revali notices something curious.
Rudania does not glare down the mountain at evil's den. Naboris does not fix her bloodthirsty grin on her oppressor. Ruta does not patrol above her domain as she did a century prior. No, not a single other Divine Beast stands poised and ready to attack.
Mipha had truly loved Link, and while she made it clear that she considered equally important the side of her that was a Champion and the side of her that was the Zora Princess hopefully betrothed to Link, she always put Champion first, given the imminent doomsday. Urbosa watched over her fellow Champions with a fierce yet motherly caring, and was always the one to assure Link that Zelda did not hate him. Daruk bantered with Link despite his silence, and never once lost faith in their assigned roles. Revali is the only one who ever questioned Link's merit or thought to challenge it. Revali is the only one who ever resented Link for being chosen as the Hero, the Champion who stood superior to all the rest. Revali is the only one who did not entirely quash his doubts, worries, frustrations, or selfish problems under the weight of preparing for Hyrule's clash with destiny.
Yet he is the first liberated. It makes no sense.
Why?
He is alone... and confused.
Revali stares out over Hyrule as the sun sets at his back and the blood moon rises once again, and finds no answers in the night.
End Chapter One
It occurred to me throughout the game and exploring the dialogue of the Champion Spirits that none of them allude to Link's lost memories or even death. Zelda has kept tabs on him since his revival, but the other Champions only address Link when he's within their beasts, so they could have access to much less information than Zelda.
And while I take great pleasure in the stories that show off Revali's immaturity, I think the gravity of their situation would suggest that in canon, he's not so immature that he would severely hinder the other Champions' efforts to become as strong as possible.