Dan is one of the Rat Bandits that is a part of the "Splinter Group" optional mission. Dan is the second 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.
Dan is an Easter Egg of Donatello from the Teenage Mutant Ninja Turtles.