I'm going to try and keep this relatively spoiler free, but if you want more explicit hints, I can try to provide them.
I just got started on this one over the weekend. As with all NIS games, I love the graphics and style, but I'm a bit confused on a couple things:
1. How exactly do characters gain exp? I've noticed that there is XP given out after a battle, but some of my characters have also levelled up during a fight. Are these the same exp, or are they applied towards different things? Do you get XP for simply performing an action, or do you only get XP from killing an enemy (i.e. Disgaea)?
Your characters gain XP if they participate in a battle, but get more if they put the hurt down on enemies.
2. Whenever I go back to the main island, I get told that things have gained mana while I am away. Is this because of something special I am doing, or does it just happen naturally?
I believe that any item that you have that is actually on the island (not in storage) gains mana proportional to it's level. Additionally, if you have recruited certain characters and they are present on the island, your items might also get a boost from them.
3. Do weapons level up with use, or do they only gain XP when they kill something?
Memory's a bit hazy on this point, but I believe that it works like this: weapons gain XP (not sure if it's exclusively kill) up to a certain point. After that point, you need to have a character level up the item in exchange for some mana. You can also level an item before its time XP-wise if you spend the extra mana.
After about 3 hours, it seems like a great game. I just want to make sure I understand how it works before I get too far into it. The problem I've had with the other NIS games (Disgaea and La Pucelle), is that I'd get 30-40 hours in and realize I'd built my characters or team all wrong. It'd be better if I started on the right foot with this one.
Totally understand the feeling. With these sorts of games, it always seems like you need to "waste" 10-12 hours figuring out how things work before you can build the ultimate team. Thankfully, developing new characters is significantly easier in this game vs. Disgaea thanks to a nice trick/flaw in the game system.