Customize, randomize, itemize and... realize?
You unlock new characters by leveling up a class like warrior or even multiple classes like warrior/brawler. You can also unlock monsters by beating them in battle and each time you beat a specific monster it costs less to buy... not that you'll be able to afford a dragon any time soon~
There is so much to this game that it is difficult to explain why it is so amazing. If you can borrow a copy from a friend or rent one I would say do that because this game, despite being amazing, is not for everyone. I'm REALLY directing that at people who like their games as simple as possible. Hero already left out.