There're a few grammar issues where wrong tenses are used. A couple places where there's dropped punctuation. I would also suggest dropping in a linebreak before and after the flashback sequence and removing FLASHBACK as well as the stray full stops you use to space the story. Those can be replaced with a linebreak as well.

You may want to stop using ellipses to denote breaks in converstion unless they're denoting a dropped word. Like "god is … dead" with the dropped word being bloody as an example. To denote a pause or a self-interruption one could use —(the Em dash).

But not knowing the universe, i liked the story for the most part though the references went over my head.

Oh, one last nitpick. Where her skin is blackened by dirt. I might have used 'layer upon layer of filth that built up upon her skin' but that's my views on the word blackened.

Hello precious,

I was in one of those be nice moods, and I came across a reddit thread in which your want of a review was mentioned, so I stumbled my way over here. I know what it's like to get an informative and (hopefully?) helpful one, so I hope that I have accomplished this!

I want to start off by saying that I started reading, got halfway through, and decided to start over and write down comments as I go through the story. This was going to be just a passing little thought-word-vomit conglomerate, but I thought it would probably be more helpful if I just wrote everything I thought. This is a mix of line-edits and general thoughts, and though I might be a bit harsh or vague at times, please don't take it that way! I hope that you don't mind con-crit. 3

Disclaimer: I am not an editor - just a bit of a grammar fanatic/nazi. All of these things are (for the most part) just suggestions, and I know that my word is not law by any means.

The first sentence is very interesting to me. You manage to set up a lot in a little: she's in a prison cell, she's bored or anxious, and she's probably alone.
-However, the double use of cell gave me a bit of pause. It's not necessarily bad per say, and completely up to you, but I just find it somewhat redundant.
Maybe something like: the sound resonated through the otherwise silent room and echoed back to her ears.
(or something)

You have already put her thoughts in italics, yet you also use a tag to denote her thoughts. Both are unnecessary; you could probably remove the italics, if you want to have her thoughts broken up by the thought tag.

This next paragraph provides a lot of interesting info: primarily just how isolated she is. The piercing screams were definitely a good descriptor, and the lack of information that she herself has is... daunting, in a way. I appreciate that!
-However, the syntax flows a bit.. stilted to me. Little changes would probably go a bit to remedy that:
-she sat in a long silence in her cell... I've never heard of silence being described as 'long' (at least, not when not actually recounting or telling a story in real-time. Maybe ever-present, stifling, something that actually describes the impact that it has on her?
-why real sunlight versus sunlight? My brain is admittedly a bit dotty, but the first thing that comes to mind is: is there artificial sun that we're not aware of?
-She could only begin to guess at the time of the day, or perhaps the night. I appreciate this sentence for what it is: showcasing her lack of surity, but I think there is a better way that this could've been written - She could only begin to guess at the time of day, if it was even that/or was it night?/something that just flows better.
-The time [in their] between meals... this sentence in general confuses me. Is it the time in the cell that is lost to her? It gives that impression, at least, and I'm sure that's not what you mean to convey. Also, the use of lots of ','s makes it feel a bit fragmented; consider using '-'s to surround the clause about outings?
-courtesy of the Fire Nation's interrogators who have been working with haste (you change tense here).

-Suki's ears perked up at the registering of a faint sound. (maybe? something that doesn't make this feel so stilted)

next paragraph: the scrapping of nails, yes yes. I love it. Such descriptor 3 It makes me tense and anxious for her.

-epithets. I.E: the ugly boy, the long-haired girl, 'the Kyoshi Warrior'. They are nothing bad, per say, and they do have their place, but most people feel that there is no need for them when the person's name will do just as well. I thought I'd just comment on that! It's definitely up to you though.

-His groans of pain lasting the majority of the night (lasted)

-despite it keeping her from sleep[ing] (that is, consider removing the ing)
That sentence is so poignant though. It speaks to me: her inability to ignore someone's pain, and the reasons are unspoken, yet so understandable.

-their daily outing in the prison yard (we already know where they go; perhaps shorten it to just 'their daily outing' - or consider changing the phrasing a bit; it's exactly the same phrasing from earlier, and thereby detracts a little.
-'Only depositing them back in their cell...' maybe to 'They would only deposit them back' or something along those lines; only would denote a subordinate clause, and yet this shouldn't really be one (I think).

-'They worked in a simple manner.' Would be fine, if not for the redundancy later on. Maybe change this sentence to something else? I.E: 'Their manner was efficient/simple.'?
-'one per night; and...' while a semicolon can come before a comma in certain instances, it is generally breaking rules, and I don't think it's necessary to do that here - a comma instead of the semicolon would probably give the reader less pause.
-'about the invasion(,) and those who escaped' (probably don't need a comma there)
-'would stop(,) and spare the rest of us when we(,) too,' . Probably unnecessary commas. Also, you go from talking about this in the third person, and then the we comes out of nowhere. It threw me.

Next paragraph: Yes. Yes. Yes! hellish cries; detached from reality; their screams, their moans; unworldly beast. I love it! Such tiny little phrases that totally jump out at me and batter my brain.
-who was beyond all hope of (salvation)

-cries: redundancy.
-'but it was a hell...' but to yet?
-'her own; but that never stopped Suki and the other...' the jump in thought-process is a bit jarring and somewhat unexplainable to me. I mean, I get that their trying to help her, but I don't know. These two thoughts shouldn't be linked. If nothing else,
'her own. That never stopped Suki and the other warriors from (etc etc), however.'

such a sweet paragraph. I reread twice to let it sink in. ;) (the one about respect)
And the following thoughts are so... jarring - beautifully so. So somber, sobering, and sharp.

[skipping a bit]

-willingly subject herself to that (why is there a line-break here?)
-was(: 'so that nobody else has to' - the same answer she would always give' (that is, add a colon, change quotes to apostrophes, move one earlier, and connect the two sentences.

Your diologue tags are fine, but you are writing diologue incorrectly, if that makes any sense.
"Bad memories. They do that sometimes(,)" Masa said softly...
That would be the correct way - in otherwords, a line of diologue is not sentencetag, it's more like wordstag sentence. (don't worry, I had to look this up recently myself; it's actually in my bookmarks!)
This might help you out a bit: /columns/talk-it-out-how-to-punctuate-dialogue-in-your-prose

-'Think about it; they have prisoners they want control...' Are you trying to say that they want control, or they want control of the prisoners? Add a period between prisoners and they if the former, control to controled if the latter.
-'(They are m)uch stronger than most of the prison guards there.' This is not an independent clause the way it is written currently.

'benders; who,' (Suki's diologue). Change semicolon to comma, methinks.

-'publically beat him; but,...' (here too)

-small, knowing(,) grin...' (remove comma)

About the flashback in general: I really loved the scene. The description of the sea, Suki's blatant innocence, and the words that you use in your diologue itself are great. This was definitely a very interesting part of your story.

-same thing about thoughts after the flashback - and thoughtstagsentence.

-leaving her feeling worn out, both physically and mentally. semicolon separates two independent clauses, and this last one would be subordinate. Either change to comma, or change phrasing of latter clause

-by some freak chance (that) the guards changed their simple pattern (remove)

-'how slim the chances of it happening were(;) and(,) although' (change to comma, remove comma)

Ok. Wow. I didn't mean for this to get so long haha, but I just got really caught up in this.

I think that this was a fascinating attempt at getting into Suki's headspace. It was such a bittersweet outlook, and I just. I mean. It's great. There were prose mistakes, yes, but not to the point where I was like 'FUCK THIS SHIT I CAN'T READ RIP MY EYES OUT JESUS', and I wouldn't have taken the time to go all r/destructivereaders if I didn't think that this was worth it. I completely understand if you get upset by me... all but tearing into your work haha, but I genuinely do this with the best of intentions. I really hope that this helps you, and I want you to know that I truly do think this was a great idea, and generally not even badly executed; I would hate for you to think otherwise!

Passing tips: What? Melody? Where do you nazi? I read, google shit when I write, read some more, and generally just get a gut feeling for these things. Practice, practice, get your writing critiqued until you hate everyone, and practice some more. Google, hate writing, and then finally, you will see what works and what doesn't. I.E: semicolons before and are fine, in the right place. etc. But just like, I will feel terribad if this makes you feel like a crap writer, because you aren't, and I just. You have gems here, and you have reminded me that I used to love this fandom, and I hope you don't take this the wrong way haha.
tl;dr of this last paragraph: I am rooting for you, don't give up!

You weren't kidding when you mentioned that this story was dark. It's a good thing though. A touch of darkness for the soul...can be a learning thing for the soul from time to time. You know, Avatar didn't explore what happened at the Boiling Rock, but torture is not too far off a thought. What kind of torture though? Perhaps it's best not to think.

This was very dark, but very beautifully written. I loved it. My heart sped up a little bit at the end.