In River City Random Ex, released all the way back it 2004, you could recruit gang bosses after you beat them and they’d follow you around like little artificially intelligent henchmen, Underground will have this feature too.
Not only that, but you’ll also be able to play as the bosses you fell with you hot blooded fists. Along with this info there’s a cool level design demonstration to watch in the most recent update.
In a not as recent update new rewards have been added that include dinner with Yoshihisa Kishimoto. Go have a Double Dragon Dinner!