Keith is full galra and looks it, tho he is still small for a galra. He is from the pre -war era, before altea was destroyed by the empires rise.
Before the war with the galra, alteans had galra and other alien immigrants on altea, keith himself was a servant in the castle, but after the war began the galra citizen rights were taken, those who didnt escape were made slaves by the alteans to keep them from joining the galran ranks in the war.
of course the galra are a warrior race and they dont submit, so they would be restrained and junk, like collars, cuffs and muzzles.
So now keith is a slave in the castle, poor muzzled keith, he doesnt know when to quit.
Keith is of course far from submissive and gets punished like alot, and when the galra show signs of winning over altea and the castle ship launches. keith ends up with it.
Alfor puts keith into cryo with allura and coran so they have a servant when they wake up and you just know keith is terrified. Like he hears the galra are winning and he believes he will be freed by his people only to get frozen and told he will remain a slave... poor keith...
So when the garrison crew come and the cryopods release keith probably falls flat on his face(cause you know alfor had to chain him up to keep him from taking advange of whoever woke them) the garrison crew are freaked about the galra, but even more so the fact he is in rags, thin as a twig and wrapped in chains.
Of course as humans they are so not okay with slavery, especially shiro who was a slave not too long before.
The story follows along as the paladins try to make life easier for keith as he fufils his duties and the alteans being spitful for them being nice to keith so they punish him more behind their backs, just angst all over the place.
Keith being restricted as a slave (maybe his collar or cuffs keep certain doors from working) can't goto the hangers, even tho he feels reds call, and the alteans are all confused by the red lions fussiness when it was proven lance could pilot her (cause lance got red out of the galra ship and they formed voltron before retiring them to their hangers)
Things get worse when they find out keith is who red wanted, cause the black paladin that betrayed altea and started the war was galra, gasp, shock, horror, ect (coran has a change of heart around this point tho he doesnt show it right away)
The story just goes on with the angst til allura finally gives in and takes off keiths muzzle like "he can pilot the lion but i wont like it!" Allura still bullies and punishes keith in the background
Mind you keith doesnt tell anyone cause hes afraid allura wont let him pilot red, so he is really submissive and jumpy.
the turning point is when they find out its the black lion the galra are tracking and allura cant subconsciencely blame keith for the galra attacks (like how could he have??? he literally has no galra friends or access to galra channels??? everything has changed after ten thousand years)
Add to that the fact they met Ulaz a "good galra" who noticed keiths cuffs and collar (like how can you not?) and took keith aside at some point during the mayhem to tell him he is welcome in marmora (free the boy please)
So while keith is debating if he want to leave (which he doesnt cause his relationship with red is super tight and she loves her fluffy son) to goto marmora or stay and deal with not being more then a slave, (like how does that work... he is a slave... but still a paladin?.. dont question it lucas... dont question it)
Since keith is a slave and was frozen for ten thousand years i doubt he'd have a marmora blade, so the trials can't happen how they did in canon. Allura doesn't really trust keith to goto the marmora base after they fix the black lion tracking issue, but the red lion is the only one that could make the path safely so she had no choice.
When they get to base keith says he want to join and shiro goes "gasp didnt see that coming" and kolivan is like "you must do the trials" and he does and shiro and kolivan from the observation room, see the pre-war altea visions and stuff and like
keith as a servant still with his father (keith is full galra in this one) and his father telling him they have to leave altea cause something is coming, and keith knowing what is coming is conflicted, cause even tho alteans enslave the galra immagrants, the galra turn worse after the war starts. so he doesnt leave and stays with altea (giving up his freedom cause he knows he can make a difference in the future)
Then the vision splits to the war and keith and the galra being enslaved on altea, and it shows the suffering and pain they went through to be subjugated cause they would not surrender. and again the vision diverts from the memory, giving keith the option of getting revenge on the alteans (like maybe it gives him the choice of killing alfor or something) and alfor is there taunting him about the fall of the immagrants who only came to altea for a new life, but again keith decides that he will let himself suffer so he can make a difference in the future after he is frozen, the vision cuts off when he is chained and shoved into the cryopod.
Keith comes to and shiro is there conflicted and the BOM crew is yelling to give up the blade , you know how the scene goes in canon, except this isnt his moms blade its a blade he was given for the trial and he isnt unaware of his heritage, he just wants to join the cool ninja club.
So yeah, red beating up the astroid, BOM crew yelling, Shiro conflicted but defending keith, and keith like "just f*cking take it" and then wooosh, it turns into a sword and the blades just stop and are like "oh... it did the thing"
so now with a shiney new sword and alot of bruises and trauma, they fly back to the castle with kolivan. Allura is still sour about keith going, realizes he joined their blade club and throws a tantrum "how dare, you are my slave, blah blah" and shiro and kolivan who saw the memory vision defend him and allura is like "betrayal"
the garrison trio is of course confused cause they didnt get any exposition on what went down, they are kinda hurt also at the fact keith joined another club while they thought he was part of their club.
later on when everything cools down allura tries to corner keith alone like she has in the past to punish him for joining the blades, but this time the garrison trio to the rescue! cause i need a reason to show them how unwelcome and in danger keith still feels, so they understand why he joined the blade.
Endgame is keith leaving for the blade after zarkon is defeated and voltron isnt needed anymore. like in this AU there was no kuron, or disappearing shiro. so keith vanishes. maybe it continues as him fighting from the blades side when lotor joins the picture but thats pretty much as far as i got.
ps. the alternate route is if keith continued as a paladin when lotor appeared, i imagine allura would be completely okay with the whole "hoktril" party, so yeah... let the boy be happy damn it!