Dennis is a lazy Diesel, he has a twin named Norman.
Dennis is a lazy diesel who will do anything to get out of working. He takes advantage of any opportunity to create an excuse to avoid a decent day's work. When he's found out, he’s sorry -- until the next opportunity to avoid work comes along. 
He arrived on Sodor to do shunting work but was extremley lazy. He managed to coax Thomas into believing the trucks weren't behaving Thomas shunted them, only to be tricked again as Dennis said he didn't know where to go. Thomas showed him the way but left him and Dennis faked breaking down and Thomas left to get help. Dennis ran away and derailed and Thomas found him. Dennis apologised and Thomas pulled him out and helped him deliver the trucks. Dennis has changed his personality and is now busy with work.
According to a magazine article, he lives at the Sodor Dieselworks with Diesel 10.
Dennis tries to take advantage of others and depend on them to do his work. Dennis will always create excuses to get out of work, but usually gets caught in his lies, and is always very sorry...until he finds another oppurtunity to get out of work again!
BasisEditDennis is based on "No. 11001", a Bulleid Diesel built at Ashford Works, England in 1949 and scrapped in 1959. He also carries his basis' number. It was designed for shunting and mainline duties with a top speed of only 43mph!
- Because Norman is Dennis' twin, Dennis may be returning to the television series soon.