Finish the game on easy but do not overwrite your save. By finishing on easy you would have unlocked two of the characters. After that, go to the game settings option and switch the game difficulty to hard and then go to story mode but select Continue instead of New Game. It will simply play the ending and after the ending credits, you will unlock the last two characters without having to replay the entire Story Mode on hard.
You can unlock arts and voices for each character in game.
How to Unlock
Buy every move for character at least once in Combo Make mode
Win 50 matches in Survival mode with character, you will get new voice every 10 wins