Grub Burger appears as an antagonistic rival restaurant in Diner Town Tycoon. It peddles cheap, unhealthy food with addictive ingredients and it is the goal of the player to help the other restaurants out compete Grub
Burger and drive it out of town.
Grub Burger's role in the story is mostly shown through interactions with Mayor Wimple.
In the opening comic Mayor Wimple is seen talking to a representative of Grub Burger who is hidden from view. The mayor states that he won't allow Grub Burger to build a restaurant in town because the voters don't want it, and the representative offers "donations" to change his mind. What eventually allows Grub Burger to push their way into town is when the mayor is offered a sample meal and becomes addicted to the highly questionable Ingredient X.
Despite having the glowing green appearance of cartoon toxic waste, Grub Burger is largely successful in it's business, having already caused multiple restaurants to shut down. It's addictive properties are shown when Mayor Wimple is found by Flo, disheveled and surrounded by multiple wrappers and he says he can't stop eating them.
When the mayor tries to show his support for the local diners the representative shows up in a limo with more food to lure the mayor away to convince him to say he supports Grub Burger and further feed his addiction.
Without the Ingredient X it would appear that the food Grub Burger serves is no good. A single burger left the mayor feeling sick.