Korra: choleric/melancholic, Bolin: sanguine/phlegmatic.
Bolin’s attraction to Korra’s raw choleric energy is immediate. He clearly needs no convincing in this pairing.
Korra also feels some attraction to Bolin. His phlegmatic secondary naturally draws her, and as said with Tahno, being sanguine is a +10 to sexy. He’s charming and fun and she’s a bit taken in by that. But not actually having any humors in common, they may find it difficult to understand/relate to each other, and then we get to Korra’s issue with people who aren’t choleric.
This was covered in a previous post, but basically, she’s prejudiced and only respects people who show choleric traits. Which isn’t to say that she doesn’t like Bolin, she clearly does, but she doesn’t feel that respect for him, and since she’s a primary choleric, that’s rather fatal to their romance. Bolin’s silly, self-effacing humor just isn’t what she sees herself with romantically.
Which is actually a damned shame, because if she let go of that “choleric = good,” they really do have a lot to offer each other. Their relationship would be incredibly comfortable and easy. They both completely know their place with each other, Korra being eager to lead and Bolin being eager to follow, so they’d be very stable. Once these two got together, they’d be unlikely to break up, unless they tried to reverse roles for whatever reason (which would be absolutely disastrous, and likely a traumatic experience for Bolin) or if Korra crossed a moral event horizon which shocked Bolin (which is unlikely, because he’s a positive influence on her morality).
Temperature-wise, they’re well-matched, both with one hot and one cold. Korra starts out hotter, which suits Bolin just fine, but he will probably be relieved when she starts to cool down and mellow. They both have their hotter humors as their primaries, so they can be quite passionate, and physical with each other.
The sanguine + melancholic crossing of wavelengths shouldn’t give them too much trouble. Korra will never really get Bolin’s sanguineness, but she’ll be charmed by it and attracted to it, and he’s phlegmatic enough to not get really obnoxious with it. Bolin is already sufficiently enamored of Korra that her developing melancholic side shouldn’t be any kind of dealbreaker for him—he’d just incorporate it into his love of her, even if he doesn’t necessarily “get” it. In other words, they won’t relate to each other terribly well, but they’ll probably be okay with that, because their relationship will just feel so natural and comfortable anyway.
Sexually, they start out in an okay place. Sanguine + phlegmatic mixes tend to be really vanilla and fluffy, and their turn-ons are things like hand-holding, saying “I love you,” and other romantic gestures. Korra won’t really get any of that, but it won’t bother her either, and she’s very physically demonstrative with her affections, which Bolin will like a lot. She’s also very aggressive, which he’ll definitely like. He’s basically your standard romance novel heroine, waiting with bated breath to be ravished. Though he won’t like actual violence during sex, physical intensity is great.
As her melancholic side develops and she cools off, she’ll probably be less sexually demanding, which Bolin will probably be okay with, because all his parts are bruised and sore. Not that he’s complaining or anything. It’s good sore. But giving him a chance to recover is good too! Not having Tahno’s intensity, he’s less likely to feel abandoned, though he’s still a primary sanguine, and therefore does need a lot of attention and reminders that he’s loved.
Her growing melancholic side may also enable her to appreciate Bolin’s romantic gestures a bit better, and even possibly return them. She won’t really feel it like he does, but she’ll intellectually understand that this is how he shows love, and that it’s important to him, so she’ll be willing to meet him halfway and take him out to romantic dinners sometimes, even though it’s not really her thing. She does like seeing him happy—and she also likes being able to manipulate him. So if she figures out that, say, whispering “I love you” in his ear during sex always makes him come, she’ll be happy to do it, because she loves the feeling of power. And Bolin will of course be amazingly turned on by this.
If that’s as far as her kinks go (and really, the amount of control she’ll have over him might be kinky enough for her) they’ll do great. If she starts to like more adventurous stuff, especially any kind of BDSM, Bolin won’t really be able to follow her there. He desperately wants to make her happy, but that’s just a huge turn-off for him. Also, he would make the worst dom ever and shouldn’t attempt it unless his goal is to make people laugh.
Really, the biggest thing holding this relationship back is that Korra thinks she doesn’t want a man like him, that she wants a man who’s aggressive and strong and will stand up to her. While she believes this, of course it’s true for her because her mind makes it real, and if she tried to have a relationship with Bolin while believing this, she’d try to force him to act choleric, which would pretty much end up in her emotionally abusing him. But if she both accepted that she actually is most attracted to phlegmatic people on a physical and emotional level, and accepted that that’s what Bolin is and that’s not going to change, they could be really good together.
By the way, I’m not saying Bolin is a “pussy” or anything horribly pejorative (or gender essentialist, or generally misogynistic) like that. He’s a very sweet, fun-loving, humorous, affectionate boy, and those things are great. He admires and respects strength and authority, and he’s got the inner confidence to be okay with that and not try to force himself into a role he doesn’t actually want because of his gender. Korra has her own issues with androcentrism and how that relates to her self worth and how she determines the worth of others, so she needs to grow up a bit and self-examine that stuff. She might be better at it once her melancholic side develops more, meaning this couple is more likely to happen (and actually work) as they get older. Though dating his brother might complicate things for obvious social reasons, lol.
I am also not saying that Korra needs to drop her own wants and go for the “nice guy.” Most of the “nice guys” go after “nice girls,” with a lot of phlegmatic in them, who are of course attracted to choleric “jerks.” Korra isn’t the nice girl, Korra is the “jerk,” lol.
Being able to accept that you don’t need to be powerful to be awesome is also really important for her development as the Avatar, and might open her up to non-benders more and make her more compassionate. Choleric/melancholics can be a bit prone to thinking that those who don’t stand up for themselves deserve whatever they get, and that’s obviously not a good thing for her to think.