Yeah, I've only played 6 hours but that's kind of my problem with the game so far, I would have liked to raise a priest or a warrior but it's useless if the main characters are vastly more powerful than them. Are the unlockable classes better ? I've unlocked the Thief but she doesn't seem very useful.
The thief has a much better steal rate than any other characters.
It can have up to 99% while the rest has a maximum of 50%, that's the main reaso,.
But it also has great movement capacity which help reaching far away/high grounds.
I can't say for the other classes since I didn't bother with any after.
That's mostly because I prefer to play with actual characters more than generic ones.
And the only things (I think) that makes a character better in a way or another is the Overlord attack and the main unique Evility (and I feel like you can share these at some point, no sure).
In the end you can pick anyone and do anything. Some characters are just going to be closer to a specific perfection a little faster than others.
It's really a matter of choice. Maybe you'll like abilities from a specific class. You just have to check what they do.
I didn't bother because the balance of the game is so easy to break that I didn't feel the need to care for the classes.
I don't think I'd go back in the series for a long while if not at all because I clearly like challenge and story more than statistical playground.
But it is entertaining in some ways especially while going to work for example.