Training Points Underflow
To set up, You need character 2/3 to have at least one training point, and more training points than your main character. To perform, enter the training menu and switch characters. Next, press up and start on the same frame. Re-enter the training menu and your main character's stats should start increasing. This glitch only works for the main character. The game does keep track of your training points below 0 (even though the counter sets the value to 0), so once you perform the glitch, you need to redo it to invest into the main character again.