If you head to the Win Contest mode, you can unlock a new, random character by winning seven rounds in a row. After the seventh successful round, the unlocking round will come about. If you defeat the character given (you don't have to defeat the character right away, you can take several tries), you will be able to thereafter use that character. Just be sure not to leave Win Contest mode while going after your seven consecutive victories, or you will reset the counter and have to start once more. Otherwise, this can be done over and over again to unlock all of the game's characters.