D-Xhird

To use Venus and Isanagi from Kabal
Finish the game normally. At the end, you will fight both of them and after you defeated them, they will be selectable.

To use Shadow Boy from Kabal
Beat everyone, (including the bosses) without continue. You will fight Shadow Boy after you beat Isanagi. Shadow Boy is Boy's alternate persenality.

To use Eiji (from Tohshinden) from Kabal
Get to Shadow Boy and beat him without continue. You will fight Eiji after that. Eiji now even has a new costume!