To answer some of the question above.
Joker was designed in the first place to retrieve Cube in the very first place.
Cube was originally RB3's power source.
Mondo had acquired Cube for some time and it was hidden in Tataros.
Dr. Klein was hired by Mondo to build his not so impressive robot by using Cube.
You should be able to connect the dots from here.
Also, if you want to make a not complicated SaGa table top game. The best idea is to throw the original mechanics out and come out with a original mechanic of your own.