Hold start when you've highlighted the character, then press any of the three punches or kicks to get a hidden costume. You must have beaten the game with a character to use any of his/her hidden costumes. To use the secret costume and press LP, HP, and MK.
Extra Options
Beat "Arcade Mode" with Gill.
Fight Q In Arcade Mode
In order to fight Q you must not lose a single round up to the end of stage 8. If you do this successfully his picture will flash across the screen and the fight will begin.
System Direction Page 10
Beat "Arcade Mode" with 15 different characters.
System Direction Page 8
Beat "Arcade Mode" with 5 different characters.
System Direction Page 9
Beat "Arcade Mode" with 10 different characters.
Unlock Akuma in Hyper Street Fighter 2
First select Super T mode. Next have your cursor on Ryu for 1-5 seconds, then do the same for T. Hawk, Guile, and Cammy in that order, the last person to highlight will be Ryu again. Once you've highlighted Ryu for the duration of time, press Back Button, then you will have less then a second to press LP, MP, HP all at the same time. If the character you have choosen has a black shadow portrait and no name you now have enabled Akuma for the battle. If you wish to use Akuma again, just repeat the process.