- "He has been late on 6 occasions! 1, 2, 3, 4, 5, and 6 occasions, and acts like he hasn't a clue why he's there. We need a competent, constant, and conscientious pilot that isn't idle, isn't Dennis, has a clue, knows his place, and ISN'T DENNIS!"
- ―Gordon, The Suggestion Box
Dennis is a perpetually lazy grey diesel shunter.
When Dennis first came to Sodor, he manipulated Thomas into doing his jobs for him because he was too lazy to do them himself. But this ending in him falling into a ditch, and Thomas had to rescue him.
Nowadays, he is smarter than that, yet he is still lazy when shunting trucks, and enjoys a good snooze in the sunshine.
When Richard Hatt introduced a suggestion box, Dennis was one of the engines present to listen to the suggestions made. One of which was to add a permanent station pilot at Knapford. The engines were enthused by this prospect, but Gordon was insistent that the pilot be anyone but Dennis, given the lazy young diesel's track record. Later on, Dennis' mind was royally blown by Charlie's philosophical comparison of comedy and art.
He is Norman's brother.
Dennis will do anything to get out of work. He puts so much effort into shirking his duties that it is almost as much work as actually doing the job. He'll gladly take advantage of any engine he can to get his work done for him. When Dennis is called out on for his laziness, he is very sorry, but when the next opportunnity for shirking arises, he'll hop right on it!
Norman, Dennis' brother, is often disappointed with his idleness and inability to apply himself, but Dennis is always there to brighten Norman's spirits when he's bogged down with work.
- Season 2: Paxton and Norman (mentioned, not named), Munitions (cameo), Hibernation (cameo), Blunderbuss (cameo), Luke and Millie's Blue Mountain Adventure (cameo), Aura of Menace (cameo), All the World's a Stage (cameo), Swashbuckler (cameo), Thomas and the Fortune Teller (cameo), Self-Fulfilling Prophecy (cameo)
- Season 3: Logan Leaves his Mark (cameo), Culdee Fell (cameo), The Suggestion Box, Penn Pals (cameo), Ice Breaker (cameo), Chivalry is Dead (cameo), Percy and Asbestos (cameo), James Goes On A Streak (cameo), Merrick's Wake up Call (cameo)
- Web Clips: Thomas the Tank Engine 70th Anniversary Celebration! (cameo), Gina gives Percy some pointers - Meet the Characters! (cameo)