Umbrea Bisafan

First of all I want to mention that it still amazes me how much work and effort you've put into that story. In the course of 1 and a half years you've basically written a fanfiction in the length of the first 4 1/2 Harry Potter books. That's not only incedibly fast (especially for such good content) but also requires a huge amount of planning (even if you create the story as you go), commitement and patience. Man, some authors could really follow your example.
So, such an extraordinary fanfiction deserves a special review, don't you think? ;) I'm not gonna focous on things like gammar, language or writing style in general since I'm a forgeiner and to be honest, English isn't really my forte. But there are a lot of other things that can be looked at, right?

Characterisation: Given the support conversations and the in game dialouge, your characterisation is pretty much on point. Some small things here and there (like Panne joining the shepherds in order to kill humans), but most of this stuff can be left to personal interpretation.So what really matters are the characterisations you've established by your own: Lucina, Robin and Chrom.
Lucina is, by far, the best written character of those three. I used to dislike her a lot in game, mainly reasoned by her stiff personality and her father complex (I'm really sorry for tht Lucina, you're a complete cutie! uvu) And well to put it simple: You've changed that. Not by adding insane amounts of backstory or made up content like I've seen some other wirters do, but simply by explaining her actions. It is completly understandable concidering the fact that she was expected to save the world on her own from a huge giant monster that traumatized her as well as ruined her entire childhood. You never made her appear heartless or cold, she just believed that the world had be saved at all costs. So when Lucina finally realised that she wouldn't have to save the world all by her own, that she had comrades and most imortand a family she could count on, all this pressure dropped from her making her open up. A+ for reasoned character developement, my friend. (Oh and I really hope you don't plan to kill Robin or something. That would kill her)

Robin: Given the fact that Robin is actually a OC, there is a lot of space for personal preference and/or interpretation. But this timeline's Robin can only be described as OP. And by that I mean COMPLETELY OP. That guy jumped of a freaking Pegasus in the middle of a battle! Such a character trait is very difficult to handle, because it could easily turn him into a Mary sue. So be careful with that. Gladly, you decided to give him a few flaws to even the odds ;) I especially like the fact that you decided to make him cruel/cold bloded when he gets provoked or angry. The time he chopped of Exeluss hand or the how he handled the Risen-Gregor (Seriously I was not expecting that. Psychological Warfare DOES affect him after all)That could easily explain why the Robin in the other timeline turned to Grima. I assume a LOT went wrong after Chrom's death that drove him into despair and hatred.
Personally, I'd have liked to see Robin failing a little moe often or at least in situations he cannot handle on his own. (Mainly because I REALLY want to see how he handles being helpless.