In Mutant Ninja Turtles Gaiden, Mr. Carlson is a publishing executive who has arranged the sale of Mikael Buonarotti's books. Since Mr. Buonarotti is actually the secretive mutant turtle Michelangelo Hamato, Mr. Carlson only does business directly with Mr. Buonarotti's representatives, April O'Neil and Casey Jones.
Mr. Carlson only appears briefly in Chapter 10: Rational Voice? Enter Casey and April!!!.