Here is a guide on how to use hidden characters.
Hidden characters
How to use hidden characters in Super Puzzle Fighter II Turbo
• Each player must position themselves in their respective characters for it to work, if you are P1 in Morrigan and if you are P2 in Felicia.
Perform the button sequence always keeping Start pressed
AKUMA – GOUKI
• P1 should position himself in Morrigan, keep START pressed and execute the command.
• P2 tiene que hacer el mismo procedimiento pero a la inversa, posicionarse sobre Felicia y realizar el comando.
DAN
• P1 should position himself in Morrigan, keep START pressed and execute the command.
• P2 has to carry out the same command but on FELICIA.
DEVILOT
• Same as Dan, but press the select button when you reach 10 seconds.
P1 on Morrigan / P2 on Felicia
Thanks to Mariox for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.