Play as Firzen
Play with Freeze and Firen, on the same team. With 1/4 health, run into each other and you will transform with an explosion!
Play as LouisEX
Select Louis as your character, and get to less than 1/2 health. Press defend jump and attack at the same time and you have LouisEX.
Play as Jan
Select Rudolf and if Jan pops up, grab her and press defend jump and attack to transform into her. You can also switch the data files.
Unlock All Characters
Get to the character selection screen. Then type "lf2.net" without the quotes. You will be able to choose from any character in the game.