Robber Imp is a Bank Zombie in Plants vs. Zombies 3, which you can first encounter at Floor 30 of Devour Tower. He puts the first plant he encounters in his sack, making it unable to do anything, then he will run away with it. If he is defeated while carrying a plant, said plant will be released.
As his namesake says, he's probaly based off a robber, the person who steals stuffs.
There is an obvious reason why he was supposed to be first encountered in Lawn and Savings Bank, which is a reference to the robbers can rob some cash from the bank.