Excellent game although a bit confusing at first. In general, everything you can buy or earn through battle serves some sort of purpose so you almost can't go wrong with buying stuff even if you don't (yet) know what it does. It'll tend to activate when needed.
If you make it to the end of the game with the text on the screen, it's very important you click somewhere within the yellow text at the bottom to get your @ medal. This isn't obvious at all and you'll otherwise not obtain it even though you completed through the entire game.
Just about anything that unlocks a medal also unlocks a character with the exception of the secret medals. Also be sure to play as each character (both those enabled by default and those you've unlocked) to get all the corresponding character medals.
If you're trying to obtain a certain item, try to think who might be able to obtain that item and play as them.
Enemies you face seem to differ depending on who you're playing as, to some degree. So do play through with at least several different classes of characters to maybe find/encounter stuff you couldn't before.
Dex is more important than probably anything else, although you'll certainly want at least some strength and non-zero armor. Most of the bonus gimmes rely on high dex stats. And it's quite annoying to be fighting a high-tier foe who just keeps dodging literally everything. Again, high dex stats are needed. Even if you've been building dex the whole game you still might find you don't yet have enough dex.
This game is very random chance based, so if there's some exact encounter or event you're waiting for, it might be a while. This is probably the biggest annoyance. And yes you will need exact things to happen so you can fight certain bosses or obtain certain items necessary to advance to the next areas. And some of these seem to have a low chance of encounter. This artificially drags out the game length.