If you want them permanently available without typing the code every time, you can edit the game's data files:
Now go summon a battalion of LouisEX fighters and destroy the arena. Happy fighting little fighter 2 unlock all characters
There are in the official version to simply unlock all characters in the menu, but you can unlock them temporarily during gameplay or permanently by editing a file. If you want them permanently available without typing
The most famous way to temporarily "unlock" characters in LF2 is by defeating them on Stage Mode. Many new players don't realize that LF2 has a rudimentary "capture" system. little fighter 2 unlock all characters