|Well I was totally wrong about COTA having "just X-Men hero and villain characters," because Super Street Fighter II Turbo's Akuma is present as a hidden character--a conditional mid-boss, apparently, but also playable via a button code. GameFAQs says the code is
"For Player 1- at the character select, quickly go to Spiral and leave the cursor on her picture for 3 seconds, then press Left, Left, Left, Left, Right, Up, Right, Right, Down. Count another 3 seconds and then press L1+R1+X. Silver Samurai's picture should disappear and Akuma will drop from the top of the screen."
but those buttons are only accurate if you haven't remapped your controls; properly speaking, the buttons to hit are LK+HP+HK.