Here are the reasons for my typings:
Korra She's gutsy to say the least; it was mentioned how in the first episode that she excels at the physical capabilities of her bending powers, but lacks the more spiritual side. She's impulsive, headstrong, and acts very within the moment, trusting her get over careful thought. This indicates Se > Ni, specifically ESxP. I don't see any signs of Fi in her mindset, on the contrary she does what makes sense to her (auxiliary Ti) and sometimes seems to follow the moral guidance of those she's close to (tertiary Fe); ESTP.
I typed her six given her overall demeanor (I mean by process of elimination, she's certainly not 1, 2, 3, 4, 5, or 9. That leaves 6, 7, and 8; I don't see any desires that would indicate core 7 though, rather her core problem seems to focus around fear, like being afraid to fight back against Amon. However she's still counter phobic given her brash, reckless, and stubborn approach to things). Has that feisty attitude that betray sx first, sp second because I don't see any so in her at all.
Mako . . . actually I'm not sure what made me say INTJ in the first place. In fact, he might actually be ISFP. I'm also rethinking the 5w6 typing. I'll have to mull on this later.
Bolin Easygoing cool guy who enjoys life to its fullest/strong willed comic relief funny guy. I don't think any type other than ESFP would fit him. I originally typed him 9, but 7w6 actually works better (he's far more energetic and gregarious than a 9 would be). Still not certain on instinctual variant.
Asami Guided primarily be her feelings and inner sentiments, very in tune with certain feelings towards others and god damn excellent at being a strong fighter in the physical moment; Fi+Se; ISFP. Also harbors a grudge against Korra and her father; something about that strikes me as tertiary Ni. Types her as a 2 because she's certainly within the heart triad, but she's in no way a 3, and 4 doesn't seem as likely (although may be possible). Is clearly a fucking sx; sx/sp specifically.
Tenzin Takes an opposite approach to Korra's mentality; considers the long term effects of decisions and calmly reflects on things, in addition to being very concerned with upholding peace among the nation. Has the calm and serenity of a 9, with the slightly aggressive 8 wing rearing up whenever he's annoyed. Is also diplomatic and a council leader, but is reserved and polite; so/sp.
Lin Beifong Drill Sergeant Nasty stereotype with a great focus on upholding order, has a will of iron, and in general is a complete badass. I'd also consider ENTJ or 1w2 as possibilities, if she isn't ESTJ 8w9.
Tarrlok Smug Snake with a slightly aggressive attitude when it comes to getting what he wants, and is highly manipulative in doing so. That's fitting of Te>Ni>Se + 8w7 mindset.
Amon "Oh hai, Imma super genius charismatic mastermind with a grand scheme that sill change the world for the better. I'll use my chess master skills and noble conviction to win over the people and bring justice to the land!" Walking INTJ 1w2 so/sx stereotype.