A Functional programming developer writes code using a style that emphasizes and utilizes pure functions. They primarily focus on the principle that every function should have an input and an output, without any hidden states or side effects. They manage data immutability, avoiding changes to the data once it's created. They use higher-order functions, recursion, and map/reduce operations, making code easier to test and debug. They also use functional programming languages like Haskell, Scala, or Clojure. Their work contributes to efficient, clean, and easily understandable code, enhancing software quality and maintainability.