Plot = 8.4: This is certainly an interesting plot, however it occasionally fails to engage the audience. For instance, Nanaki's reasoning for not wanting to earthbend needs to be eased into a bit slower. The audience wasn't wondering what happened to her mother because it had never been an issue before, and thus the 'effect' of such a reveal was significantly reduced.
This leads the plot to move faster than it should have—another instance is the abandonment of Taising. While it's true that it doesn't take long for disillusioned followers to abandon their leader, in this instance it really was just too fast. Nanaki essentially mastering earthbending in the total of a single chapter is just too fast (and I'll elaborate on that a little more later).
That being said, the grand idea is really good—it just needs to be fleshed out and justified a little more.
Characterisation = 8.3: The characters each had their own unique personality, and with a story like this—so focussed on the Avatar's journey—it's very important.
The deduction for this comes from the author's habit of introducing characters halfway through the story. This is a big no-no, as it makes the audience feel as though they weren't planned and were a simple deus ex machina—a means to an end. This is not something you want in a story, and definitely not one as character driven as this. To avoid it, just mention the characters before the audience meets them. One the really jumps out is the sudden introduction Kozin; until that point, the details on the Fire Nation had been more glazed over, rather than presented to the audience as an event that warranted our attention. This issue also has another side; occasionally characters seem to just disappear, never to be heard of again. What ever happened to Taising? Where was Otoshi during all this?
Another aspect is the lack of characterisation after a character has been established. I see little in common with the Jinora of this series and the Jinora of LoK. Lack of characterisation occurs most commonly when the readers aren't given enough sight into what the character feels, or their motives behind it. Why did Jintaro have such a change of heart? The audience sees it, but we don't always understand it–especially in the case of Jintaro. Here is a father who pampered one twin over another, and yet suddenly after Nanaki's life is threatened, he cares? We need to see more of why he changes, not just the reasons that prompt the change.
Believability = 8.1: What I really enjoyed about this story was the setting. Even seemingly unrelated aspects can help all these categories, and for me, the description given really enhances the believability. Why? Well, simply put, it makes the entire world just that much more believable.
My notes here come from the way certain elements come together. For this story, its how fast the events take place. For someone who has refused to earthbend (and was explicitly instructed not to) after traumatic events in the past, Nanaki is a prodigious earthbender. This is a common theme in many stories—along with stories of Avatars who cannot bend to save their lives—and becomes a bit overused. One other aspect was the lack of justification for certain things. This only occurs a minority of the time, however when it does, it can be rather disconcerting. How is Jinora positive that the man whom Nanaki met with was the orchestrator of the Unbelievers and the Fists?
There are smaller elements to this story that prevents it from being as good as I know it can be—where are the spirits that everyone seems to love/hate so much? Where has the sudden idea of nuclear weapons come from?
Technical writing = 8.0: There are a few run-on sentences that need to be broken up by a full stop or semi-colon. On occasion, a comma is missing from the sentences. I also noted a misuse of a few words, the most notable being 'awhile' instead of 'while', 'whom' instead of 'who' and 'to' instead of 'too'. I only noticed one spelling error throughout the entire story, though, so well done!
There were some common nouns capitalised that shouldn't be (such as 'bow stance' in chapter eight), and some proper nouns that should be but weren't (such as the 'Mister/Mrs/Miss' titles in a few of the chapters).
Several times, I found the tense shifting between chapters, and within chapters. This issue did resolve itself, and only occurred a couple of times, however it was still disorienting at first. Aside from that, there was nothing significantly amiss—it was just little things that added up. All in all, your technical writing is rather good.
Non-technical writing = 8.6: The things I've pointed out here have really been discussed in the other categories; some things happened too fast, such as Nanaki finding out she was the Avatar, and other events weren't given enough explanation and/or justification. I feel like the explanation of how Jinora found out was half-hearted at best, and more like an afterthought. The reader needs to be eased into the story, and at that point, we didn't know the characters or their relationships with the others well enough.
That being said, there were some really beautiful descriptions, and this is one of the only stories I've ever read to use such fantastic literary techniques. I've noticed several instances of metaphor and personalisation that many stories lack, and this style is completely consistent throughout the story. That really is an achievement, and really made this story enjoyable.
Organisation = 7.8: In chapter eight of the second book, Himaru mentions that Korra had opened the spirit portal, reintroducing the readers into an established even in the series. This was not something I had thought about before, mostly due to the fact that spirits are not prevalent in this story at all—actually, they were essentially non-existent, being only a reason for the Unbelievers to hate Nanaki.
There are also inexplicable leaps in technology. One reason I believe there are no guns in the Avatar world is that, simply, there is no need for it. Not when earthbenders and firebenders can use their bending. The readers are told, quite suddenly, that the bad guy now has access to guns and weapons and is developing nuclear armaments, which does not match up with their expectations
Total score = 8.2
My advice: Get into your character's heads! Particularly with original characters, the audience has no prior knowledge to fall back on. Make sure we understand, truly, who the character is. The only other thing is to work on foreshadowing—mention, if only briefly, an element that will be introduced later on. That way, leaps such as new people, motives, and technology don't catch readers unawares.
Why I enjoyed this story: My favourite chapter was the very last one to be released—the last chapter of the second book. The way the character's reflected on the past was really creative, and a fitting ending to that part of the book. I love getting into characters' heads, and that mode was absolutely perfect. Throughout the story, such fantastic description was used that it naturally made me want to keep reading, if only to discover the next interesting way you talk about something. Keep that up, because I really think it's one of this story's defining features. You're a pretty solid writer already, so don't stop now!