Duncan is an often tempermental and careless engine, but over the years has grown up to become far more responsible and hard working. His infamous "rock 'n' roll" phase was brought about partly by his short 0-4-0 wheelbase, which caused him to ride roughly at high speeds. He is quite a powerful worker despite his size. His persona as introduced in the fourth season was consistent with his early book appearances.
Since the sixth season, Duncan is quite often bossy to the other engines about getting their work done quickly and thinks he knows better than them. Duncan still seems to ignore Rusty's warnings and advice until he lands himself into trouble. Also, Duncan tries to do the jobs which he thinks suits him well and he is quite competitive with James.
He also appears in a cactus190706 YouTube model movie called Sodor's Long Forgotten Secret as the primary antagonist, but he became the main protagonist along with the Skarloey Engines.