I'm thinking a Villain!Yuu kinda? But not like a Yuu who's mean and evil and all that. Disney villain based Yuus are great but consider: A Yuu who just... doesn't understands the logic that operates in twst. Think the aliens from Alien Stage, Mizi's guardian more specifically. (I think the alien's name was Shine?)
I think Shine somewhat cares about Mizi compared to the other guardians right? Except Shine still ends up being harmful to Mizi during her childhood. Except I don't think Shine was being malignant towards Mizi when they harmed her? To me there's like this very weird inability to understand each other in their dynamic. (I only read some of the comics, like bits and pieces only so take this interpretation with a grain of salt ye)
So I'm thinking a Yuu who just has that kind of understanding issues with the twst cast. A Yuu who is benignly harmful to the twst cast.
Because Yuu, who's from another world, who operates on a completely different scale of logic and common sense. You could take advantage of that so much in fics like. What if there was a Yuu who lived in a place where crime was common like a detective novel or smtg idk (there's always a murder mystery like what??) or idk what else but yea basically where crime was so common that you wouldn't blink if there was a murder down the street.
And Yuu, who was never taught the social structures of twst and was just thrown in some dusty haunted rotting shack (which I'm still very iffy about), was supposed to act like and pretend to be a denizen of Twst. For all we know, things that are normal for Yuu could be considered sociopathic there or smtg we never know.
Naturally, Yuu would probably take the best course of action to them. And say, we assume that Yuu tries to be good here. Except Yuu's definition of good is completely different from the 'good' in twst but they don't have a choice bec Yuu just doesn't know how twst works as a social structure.
So Yuu would probably just assume it's like back where they came from. And just, acts on it. Or, if our Yuu notices the misunderstanding, they'd learn from their surroundings. Which was a castle full of the most irresponsible and emotionally constipated people with a pretty toxic environment. Horrible example really.
At that point, we'd probably get a Yuu who just takes everything said to them at face value. We'd get a Yuu who actually tries to behead someone on Riddle's behalf or rip someone's teeth out for Leona or takes out an Octa kid out of 'benevolence' like Azul and the Tweels (bec of how much they insist on their kindness) or actually hunts someone for sport cuz Rook mentioned it off hand.
And the twst cast would just be unnerved, especially if they notice that Yuu was genuinely just trying to be nice. Because there's this gap between them in terms of common sense here.
A Yuu who fits in NRC because of NRC, a benign evil.
(Do you think this would make Crowley actually try to find a way home for Yuu? Bec their existence in there was just so obviously wrong? Probably not)
Idk it just sounds interesting if we got a Yuu who told everyone that they're from another world and the cast be like: 'Ok it finally makes sense' simply because of how alien Yuu naturally was. Like the exact opposite of the usual 'I don't believe you haha' kind of thing.