How do you get to the underground in Custom Robo Arena?

Answer 1

first you need to get the dark commanders band from your shadow self, then go to the lower left house in the harbor district and talk to the man there. you can now go into the underground from there.

Answer 2

At the end of the robo cup competition, win 11 grudge matches. then go to your team's club house and you will see a shadow there, talk to him and you will get a dark commander's badge, then go to Harbor and enter the house in the bottom left, talk to the man and ........ well i don't know what happens next..