The player "JustOwnin" and friends found a way to unlock all the characters. It takes about less than 5 hours but you dont have to do anything but let it sit there and you just wait. Watch tv or something.
Just watch the youtube video he made and it will all become clear. https://www.youtube.com/watch?v=UIBa67NGqmM
If you lack a turbo controller, or you just can't stand his voice, theres another way of getting major SP in a short amount of time.
Go to 1P vs 2P, set the handicap all the way in either one's favor.
After this, simply grind down the weaker character over and over again. You'll get about 1.2-1.8k per match and its a good way to supplement your total.
I personally grinded my way (mostly, I got about 10k from the second process) from 0 to 999,999 using my kankuro online, so theres always that option.
If you played through story mode, you can unlock most of the characters regardless (Barring Lars, Minato, Killer Bee, and maybe one or two others who slip the mind)
~Any questions? Post below and I'll try to answer, or send me a pm.