- "There's good in him somewhere, I assure you."
- ―Edward, Perhaps He's Got a Corset
Diesel is a devious diesel shunter who despises steam engines.
Diesel was the first diesel engine to come to Sodor and certainly created a foul reputation for himself.
After spreading lies about Duck and being sent away, he was repeatedly called upon to help as a last resort. Some of the events that occurred during his various visits was pushing a train of China Clay into the sea, falling off a pier and into a barge after failing to prove his strength, and being nasty to Fergus when he was sent to help out at the Cement Works. Since these mishaps, the Fat Controller had no choice but to permanently enlist Diesel to the fleet. Although he still continues to cause trouble, he is a very efficient worker.
Due to a personal vendetta, Diesel vied to have Luke sent away from Sodor. This ultimately backfired and The Fat Controller soon dealt with Diesel.
Diesel is notorious for his dark conspiracy theories. One day, when Stanley was feeling down, Diesel filled his smokebox with foreboding and tragedy about the future. It was not until Edward cleared things up that Diesel's foul play had left Stanley for good.
During the Munitions Incident, Diesel wanted to take charge and show the The Ministry of Defense Complex that Steam Engines were obsolete and that diesel power is far superior. However, Derek and Norman took none of his nonsense and "conspiracy theories", and continued to work as they saw fit. Diesel was furious when Arthur burst into the scene, but kept his mouth shut when the blaze had been caused by none other than the reckless General Zen (or so it seemed!) When the fire broke out, Diesel escaped with Norman but left Derek behind when he overheated. He was thankful that everyone was rescued in the end, and had a new-found respect for BoCo after the tremendous sacrifice he made.
Diesel was cross with Bear for waking him up on a frigid morning, telling him "to run off with the animals where he belonged."
Diesel now works at the Kellsthorpe Refinery shuttling oil and supplies to other parts of the line.
Diesel has had a conversation with Derek discussing how the steam engines, Bill and Ben in particular, had never changed or suffered the consequences of their actions. While Derek was able to confidently refute Diesel at first, Edward's arrival to inform them of Donald's accident enabled Diesel to expound on this subject yet again. He however, admit that he felt that he was an outcast and that he did exaggerate on his outbursts, but to him, it happened, and it wasn't fair. He then stated that to get back at these injustices, they would wait for the natural cause of things.
During the meeting at the Shunting Yards concerning Donald's accident, Diesel began yet another one of his tirades, tearing into the issue with malice, mockingly putting down Donald and Douglas, calling Donald's accident "good riddance to immigrants", sparking the ire of all of the engines as a result. He asked why Derek's accident was ignored, while the world seemed to stop for the Scottish Twins, ignoring the fact that the magnitude of the accidents greatly differed in scope. He proclaimed that diesels went beyond the concept of being revolutionary, and were the true future, calling the steam engines "nothing". He even managed to successfully rile up Edward on the matter concerning Bill and Ben, even winning some supporters in the matter. Before the meeting officially began though, Diesel left, mockingly asking everyone to tell him how it went. Edward admitted that Diesel had brought up some good points, but they were buried under a great deal of maliciousness.
Diesel is devious, rude, cunning, and sinister. He believes that Diesel engines are superior and holds a bitter resentment against the steam engines on the North Western Railway. He tries to keep a firm hold over the other diesels, often telling them what to think and do. He is filled with conspiracy theories and cunning plots, but they rarely work when put into action and often land him into trouble. With all said and done, there is some good in Diesel somewhere, but it is very, very hard to find. Diesel himself admits to feeling like an outcast, and wishes to get back at those who have dealt injustices to himself and the other diesels, even admitting that he does exaggerate, but it doesn't make him wrong.
- Season 1: Lift Bridge (cameo), Redemption (cameo), Feeling Lucky? (cameo), Improvisation (cameo), Feeling Lucky? (cameo), Snow Blind (remastered version cameo), Wilbert the Lumberjack (cameo), Will Power (cameo), Conspiracy Theory
- Season 2: James Goes on a Trip (cameo), Rock-Star, Munitions, Hibernation, Swan Dive (cameo), Aura of Menace (cameo), Swashbuckler (cameo), Thomas and the Fortune Teller (cameo)
- Season 3: Perhaps He's Got a Corset, Derek and the Two Faced Engines, The Suggestion Box, Penn Pals (cameo)
- Web Clips: Mike and Frank Banter: SLOTLT, ScarletFire, and EE93's Return (cameo), Thomas the Tank Engine 70th Anniversary Celebration! (cameo), Walter Sliggs - Meet the Characters! (does not speak)
- In Conspiracy Theory, Diesel had a Cuban accent.
- His 2013 model was used from Swan Dive to Swashbuckler. From season 3 onwards, he goes back and forth from his 2013 model and his 2011 "Oil-Covered" model to give him a variety of expressions.