Devious Diesel is a black diesel engine. According to the song "Will You, Won't You", He is the first diesel to arrive on Sodor.
Diesel is, in essence, the average pantomime villain. He is oily, malicious, devious, scheming and full of ideas to get revenge. He views himself and any other diesel as highly above the steam engines and believes that all diesels are "revolutionary" - he has also claimed diesels should be "rough and tough", as an excuse for bullying the other engines. His antics give diesels on Sodor a bad reputation, which causes most steam engines, particularly Duck and Thomas, to mistrust him. His rough demeanour also affects his fellow diesels, who are often victims of his tricks alongside steam engines, mainly diesels who are friendlier to steam engines and are just nice in general, with Paxton being a prime example.
However, despite his antics, Diesel still wants to prove himself to be a really useful engine. He is also very proud to be the first diesel on Sodor and seems to have a soft spot for cute ducklings.
In recent episodes, Diesel is portrayed as a typical bully with a lack of forethought. While sinisteringly cunning, he usually neglects to think of what would happen if things were found out and due to the transparency of some of his plans, he always gets his comeuppance in the end.
The Railway Series
Thomas & Friends