Yugo is a character I really didn't expect to have much to say about (not saying I don't like him, just that he's a shounen protagonist in a children's cartoon and I'd forgive them for taking the unsubtle route) but he evolves a surprisingly lot through the show, so here's all the stuff I've noticed.
First of all, Yugo is someone who is shown to be well-liked in his village. He has friends there, Alibert & the villlages love him, he's not socially isolated in any way. This is relevant because midway through season 1, Yugo tells Amalia that "sometimes, [he] feels like his home is amongst the stars." That means that while Yugo does not suffer from any social alienation from his peers, he inherently feels like something sets him apart from others in a fundamental way. He feels like he doesn't belong here, and that feeling was not one imposed on him by circumstances. Which makes sense, he DOES have wings so he's probably figured out early that whatever he is doesn't quite fit in the whole "twelve-ish gods" thing.
This leads to his... fascination, with anything related to eliatropes. I wouldn't call it an "obsession" because he's not nearly as bad as Adamai on that front, but it is something very important to him. That's why Qilby could so easily lie to him in season 2, that's what the Eliatrope dofus resonated with when he accidentally created the Eliotropes, and in season 4 he outright admits that was his personal quest all along. Dude wants to be with people who are like him. Who can understand him.
That is, unfortunately, impossible, because Yugo is the only Eliatrope who was actually raised in the world of twelve. While everyone (Adamai, Qilby, Nora) argue that he is an Eliatrope he needs to abandon this world he should come with us we know better we're our own thing, well... Yugo doesn't want that! That's his world too! He's grown here! He has people he loves here!
Yugo feels inherently at odd with twelvians. But he doesn't belong among Eliatropes either. Whether he realizes it or not, this was probably part of why Yugo was so quick to recognize Qilby's betrayal in season 2; there was no "there must be a mistunderstanding!" No, Yugo saw the facts, and realized that this Eliatrope glory he'd been shown had always been too good to be true.
(Another reason why Yugo was so quick to turn on Qilby is because season 1-2 is all about Yugo fighting minor antagonists who fucking lie and manipulate people to get what they want. See: Moon Island. See: the Black Crow. Also because Yugo grew up with Ruel, who apparently used to fake heart attacks to not pay the bill at the inn, and that probably didn't help the trust issues. After Qilby's betrayal, Yugo only grows worse on that front, to the point that in season 4 when asked about the Necros Yugo is quick to say no we can't reason with them let's just fucking fight.)
Though, despite his Complicated relationship with the Eliatropes, Yugo DOES want to be with them. Which, in his case, means "being their king." And boy, does Yugo internalizes HARD the whole king thing. He wants to be a good king, a hero, someone who saves people. Which is a lot of pressure on a guy who should be fucking around in college rn. Dude is trying to save Everyone and solve Everything and it's only contributing to making him worse. No wonder he's so much more antagonistic/bitchy in the OAVs, dude is a pressure cooker reaching his limit (AND he presumably has the worst dysphoria known to man due to being an adult in a kid body. If people kept treating me like a child as a full adult I'd be killing people with hammers personally.)
This impossible standard he's holding himself up to is also problematic because... that's the moment where Yugo starts to connect the dots on the fact that he might not be that good of a person, really. Yugo sees himself in Nox and Qilby, as people who are willing to commit atrocities for the sake of their families. Yugo threw Qilby in the White Dimension, an act that explicitly haunts him as one of the most horible things he's ever done. The OAVs had Yugo being told "hey don't do that you'll destroy the world," and then doing that, and indeed nearly destroying the world. Season 3 has Oropo, which as a part of Yugo is the living proof that Yugo is capable of horrible things. Yugo is fully aware that he is THIS close to becoming one of the very monsters he's been fighting all his life.
(Speaking of Oropo, while we don't know much about the Eliotropes, all those we saw in the Oropo OVA Fucking Hated Each Other, ranging from "thinking the others are bitches" to "outright murdering multiple other people." Since they're all parts of Yugo. Uh. How's that self-loathing doing, homie?)
The one thing that keeps Yugo going is the belief that there was no other choice. That he's only ever taken the best course of action, it's just that the best course of action sucks. It's a very fragile belief Oropo is VERY good at shaking however. That belief is also probably part of why Yugo is so antagonistic towards Qilby in season 4; because if Qilby IS trustworthy in this specific situation, if Qilby COULD redeem himself and do good, if Qilby isn't the monster Yugo believes him to be... then Yugo inflicted unspeakable horrors upon the man for no justifiable reason. Then Yugo Fucked Up Bad.
Yugo kinda gets over his issues at the end of season 4, when he stops wondering about right or wrong and instead just go full for it, arguing that "everything is fair game to stop this." Which... might not be a positive development lmao. The villain of season 4, Toross Morgal, is very much the vision of what Yugo could become. Like Yugo, he has all six dofus, like Yugo, he meant well, like Yugo, he is a king. This man outright tells Yugo "be careful not to become like me btw hell is paved with good intentions" so the fact that Yugo dismisses it is! Worrying!
Early in season 4, Oropo tells Yugo that he's seen the future- that he knows that Yugo is going to fuck up massively, and maybe it won't be by choice, but the damage will still remain. And I believe it. Everything in Yugo's story, in Yugo's environment, is building a road where he is bound to become the very thing he'd feared for so long. And I, for one, can't wait to see it <3