Easily one of the greatest hack-'n-slash games of all time.
It's a pretty generic story, really, pitting you against evil to save the world from darkness. Where the game is really strong is the gameplay and character development system. It really brings together a lot of different stats to make your character very much your own. You can even pick which color your avatar is in the beginning of the game.
As I said before, the gameplay is primarily a hack-'n-slash type gameplay. The game takes you all over the map of this imaginary world you are trying protect, and it has a beautiful area select screen for a GBA game.
Overall, if you're looking for a GameBoy Advance game to really last you a while, you should most definitely pick up Shining Soul II.