In the north corner of the Psy Academy, a person in a blue cape can be found. He is Guy, the Hero's friend, who is challenges the Hero to a sparring match. It doesn't matter if you lose or win, and these battles have no influence on the story.
There's a guard in Cleo's Bar that will play dice with you. A random roll decides who takes the winnings from bets that range from 5 to 100 coins. The best strategy for getting the most money the fastest is to save outside of the bar and wager the highest amount of money you can. If you lose, simply load, and if you win, save. Rinse and repeat.