Fight your way to N. Boss in the regular mode. When you get to him, let him beat you both rounds. When "Continue" appears, let the time run out. Now, enter the Vs. Mode. You will see N. Boss available for play as Player Two. You can keep playing N. Boss as long as Player Two doesn't try to select another fighter. Once that happens, N. Boss disappears from the selection screen and you have to do the code again.