Whit is a human that lived in feudal Japan, he is an ancestor of Casey Jones who like his descendant ends up helping the Turtles despite anti-heroic tendencies and he also shows a romantic interest in April, just like how Casey does.
He is an English mercenary working for Walker, but was locked up for trying to start a mutiny against him. He is rescued by the Turtles when they arrive to save April, and Whit later sides himself with the Turtles and the rebellion. After being threatened by Walker and Niles, he decided enough was enough, and finished Walker off by activating a catapult at Walker knocking him off the dock falling to his death.
Whit was played by Elias Koteas, who also played Casey Jones in the same film.
Teenage Mutant Ninja Turtles III