Re: question YES gldjfkfj we'd be here all week...
Look, Bucky didn't exactly have any of this in mind when he decided to help Elsa in the first place. It was only about her looking lost and needing someone to show her around- and, you know, he might not be the very model of a great citizen, but at least he can handle hierarchy in the army, which means he can handle hierarchy outside it.
And a queen all alone without any of her guards, well, that would be probably really scary for her. And he happens to have a protective streak a mile wide, so feeling useful when he could help her is what made him stick by her, initially.
Of course, then things like the vines happened, and he learned more about her, learned that they shared similarities - her protectiveness over Anna (of course, he didn't figure exactly who she was talking about, or why, until later on) mirrored the concern he had about Steve, when they were kids, and he admired her for it, which added to the respect he felt for her already- the concern she'd shown for her kingdom, the way she'd been keeping it together even when arriving here and everything being strange, well, how could he not respect that? All of that is a show of strength, and that's important to him.
And then they got hit with all the memories, and suddenly, he knew exactly how she was when she was smiling, laughing, how she was when she was angry, annoyed, closed off, stressed out. He knew her, and no matter what, he can't tell himself that's not how she really is.
Now, of course, Bucky is the master of being completely ignorant of his own feelings, as well as shutting said feelings away because they're inconvenient, and with Elsa, it's a combination of the two. Partly because it's so easy to tell himself any lingering feelings he has are because of the memories, which weren't real, so nothing he feels is real either - and partly because he just entirely refuses to think about it. He's promised himself he'll protect her and make sure she's alright, but that's because she needs it, and he can't trust anyone else to do it. That's it, the end- as far as he's concerned, anyway.
But of course, his actions betray him anyway: had it been anyone else to freeze Steve, he would have flown off the handle, but it was Elsa... so it was obvious she didn't mean it, and so what would have been anger just switched to worry. She's the one to cause the entire blizzard, and yet he's concerned about her, about how she's doing. He believes in her, is the key, that she's good and that she can get over her fears that caused the whole thing. He thinks it's because he knows her, but in reality, there's absolutely no logical reason for him to think like that... because his reason for thinking like that is not at all based in logic.
If asked, he'd just say he's like her guard and that what's most important to him is that she's a) happy and b) safe. That's it. He wouldn't even count himself as her friend, because she's a queen, right? And he's just a soldier. Another reason why he's shut off everything he might be feeling about her, because he's not nearly good enough for her, in his own opinion.
... and yet, he tends to forget even calling her "Her Majesty", now, instead calling her just Elsa, which, again, his actions are what betray how he thinks.
But if they were to get together, I completely agree with your opinion somewhere there above that they'd be a good match- he'd make her laugh and balance out her seriousness and tendency to stress over things, and she'd give him stability and yet she has a strong personality to match his, and she probably wouldn't put up with his stupid issues, whenever they'd surface. Same for him with hers, really.
Now, for you: would Sebastian reveal that he's a demon, and if, how/when? And if no, how would he react if people were to find out?