To unlock a secret area of the game called The Underground, you must first win the Robo Cub and then win twelve subsequent Grudge Matches. You must then head to your school and fight your doppelganger outside. This battle must be won.

From there, you will get an item that grants you access to The Underground, but you must first find the entrance to the Underground. The entrance is located in the town over from your hometown (to the left). Talk to the man at the southwest section of the town, and he'll grant you access.