We'll show them together v2

For a picture made in MS Paint, this looks mighty impressive. As a picture in general, though, it could use a lot of work.

Zuko's right hand for example (his right, not ours). It doesn't look like you left any space for most of his middle finger, which is why it's just peeking out at the top like an afterthought. The result is that his hand looks mildly deformed, so you might consider redrawing it or at least positioning it so it doesn't look like a claw.

While I'm on the subject of Zuko, his upper right arm appears to be shorter than his left. He also appears a tad more muscular than he did on the show, and his tricep should be scaled back slightly so it's in proportion with the rest of his arm.

Something is very wrong with Katara's nose. This screencap shows her with a similar expression, so I hope it helps. Notice how her nose is made up of two separate shapes instead of one. That's one way her design differs from Zuko's.

Also note that the screencap shows she has beads connecting her hair loopies to the hair above her brow. You seem to have forgotten them, so it doesn't look like her hair can physically do that.

Other than that, picture looks great. I like how they look ready to kick ass, and I feel sorry for whomever they're glaring at.