Dennis is a lazy diesel engine.
When Dennis was new to the railway, he tried to take advantage of Thomas. He soon learnt his lesson when he reversed into a ditch. With Thomas' help he was able to finish his work and promised to be a Really Useful Engine from now on.
In The Great Discovery, Dennis and a number of other diesels waited in the quarry yard to hear if Thomas had been found in the days leading up to the reopening of Great Waterton.
Dennis is considered to be a very lazy diesel who tries to take advantage of others and depend on them to do his work. Dennis will often create excuses to get out of work, but usually gets caught in his lies, and always says sorry - but whether or not he means it is the question.
Dennis is based on No. 11001, a prototype Bulleid diesel built at Ashford Works, England in 1949 and scrapped in 1959. He also carries his basis' number. Norman is also a member of this class.
Dennis is painted grey with dark grey wheels, gearboxes, roof and front. The number "11001" is painted on his sides in white.