Ralph is one of the Rat Bandits that is a part of the "Splinter Group" optional mission. Ralph is the third bandit that comes out when you place the pizza on the table, then disappears once his health gets to halfway. He will reappear once all four bandits have shown and had their health reduced to halfway.
Ralph is an Easter Egg of Ralphael from the Teenage Mutant Ninja Turtles.