In Marvel vs. Capcom 3: Fate of Two Worlds for the PS3 and Xbox 360, you can unlock Akuma from the Street Fighter series as a playable character.
To unlock Akuma, you just need to gain a total of 2,000 PP (player points). You will accumulate PP while playing through arcade mode, and it should only take about one playthrough to get the points you need to unlock him.
If you’re strictly trying to unlock characters quickly, I would suggest playing on the easiest difficulty, since the matches will go faster and you’ll be able to pull off more combos, giving you more points in the fastest time possible.
Check out my other guide if you’re looking for a complete list of unlockable characters.