Lots of info here (I've played for over 4 hours now). Feel free to put in the OP if you feel it would be helpful.
Links to my playthrough of the prologue: (about 100 minutes total)
https://www.youtube.com/watch?v=k1VBXGww5UE
https://www.youtube.com/watch?v=VD72jowQUsE
The demo goes beyond the prologue with 4 more missions. Nothing in the prologue is particularly long or hard (despite my troubles in the above videos, lol), but the story is actually really awesome. The music is fucking incredible, too. The 4 missions (well, the last 3) after the prologue are more "real", and are longer and more difficult.
There is a LOT of customization in this game. I will try to explain it as I currently have seen it:
-You have two levels. One is a Defense/Health level, and one is an Attack level. You level these up by saving (Defense/Health) or sacrificing (Attack) enemies. Bosses/bigger enemies give more EXP. Also, some enemies will lower your EXP in the department you don't choose (for example, saving: Defense/Health +4exp, Attack -2exp), but it will never level you down.
-You have the sacrificial magics. These are obtained by clearing stages (and possibly picked up in stages themselves?). Some drop more rarely than others. These involve all sorts of attacks, defensive maneuvers, evasive maneuvers, heals/regens, and more. As an above poster stated, if you get two of the same one, you can "combine" them for a +1 sort of thing. After that, you need 4 - then 8 - etc, to keep going to +2, +3. This usually lets you use it more times. (Using a sacrificial magic TOO many times and it will stop working completely and you need to "rejuvinate" it outside of missions by usings the books tears. As long as you don't use it ALL THE WAY down, it will fully repair after each fight. Also, you can find areas in each map where you can restore your "uses", although these are not unlimited. The proper use of each number of magic at the right time is VERY IMPORTANT to playing well.)
-There is also synthesis to make sacrificial magics. Currently I have a list of about 12 different magics that I can make, using other magics. I'm not sure yet how to find more recipes (there may not be more in the demo).
-After the prologue, you also get the ability to customize your arm. This is sorta like your "armor", although a lot of this customization grants bonuses to attack and stuff as well. You get these "seals" to place in your armor (I think it is 5-6 slots) by killing a certain number of enemies. For example, kill 5 Jack-o'-Lanterns (a boss), and you unlock a certain seal. I LOVE this system because it feels like even if you don't get the rare sacrificial magic drop you are going for, you are always making progress on your seals (as well as your character levels, of course!).
There is other customization in there as well. I'll dive back in in a bit and check it out.
