Many years ago, Diesel 10 was an extremely cruel engine, intent on destroying steam engines, especially one named Lady. Many of the steam engines on the railway knew of how cruel and malicious Diesel 10 was, and despised him for it. After Diesel 10 nearly managed to destroy Lady and Thomas, he fell off a viaduct and landed in a barge, where he was then managed by the Fat Controller. He sent him into exile for his cruelty and he had to work at the Sodor Logging Co., with Carlo Debris. While he was there, he saw the true anger and meanness in Carlo Debris and after many years he began to see the error in his ways, and became a more helpful engine, wanting to mend his reputation and be really useful. Still, many of the engines of Sodor do not know this and unfortunately Diesel 10 still has many enemies that believe his motives are the same after all these years.
Splatter and Dodge are Diesel 10's goofy associates, but not by choice! He calls them "Splodge" collectively, as the warship claims it is too time-consuming to say both names. When the three got into serious trouble in the past, Diesel 10 was exiled to the lumberyard under the jurisdiction of Carlo Debris.
After the events of "Splatter" and "Dodge" his old associates were leased to the Sodor Logging Co., and Diesel 10 greeted them upon their arrival. Lonely in his exile, he is delighted to be reunited with his former underlings.
In "Snow Blind", Diesel 10 shut the bumbling shunters up when they began to undermine the Lumberyard's precieved influence over the Ironwork's thugs.
In "Rock-Star" when Splatter and Dodge begin to make rude, silly remarks about the steam engines on the North Western Railway, Diesel 10 shuts them up for acting so idiotic.
Diesel 10 was sent to help remove the blockage impeding the Flying Scotsman's progress. Upon discovering the blockage was made up of rock, he suggests using Thumper for the task, as he might be more suitable.
While he admits to "hate" the famous engine, he nonetheless helps him out for the chance to be really useful.
During his first visit to Sodor, Diesel 10 pursued Lady with an intent to destroy her based on his premise of diesel superiority and with the intent to destroy all the steam engines once and for all. He manages to crash Lady, but fails to accomplish his task of destroying her. This incident left him bitter and anguished, creating a desire for revenge. When Lady eventually did return to Sodor, Diesel 10 once again pursued to wrap up his unfinished business. He once again fails, this time running off the end of a broken viaduct, ending his pursuit of the private owner engine.
After his failed attempt to destroy Lady, the Fat Controller exiles him to the Sodor Logging Co. to work under Carlo Debris. Carlo's harsh and dominant personality quickly proves no match for Diesel 10's own devious personality. As such, 10 has developed a dislike for the foreman, a feeling which is held mutually by both.
During a meeting with thugs from the Sodor Ironworks, Diesel 10 is sent along with the Lumberjacks to "even the playing field" against Aura's men. Diesel 10 also serves as his messenger, reluctantly delivering Carlo's hesitant sentinents regarding continued weapons smuggling.
In "Rock Star", Debris sends the "Sulking Iron Wretch" to help remove the blockage on the main line near the blasting site. Carlo cites the reason for this allowance to leave the Lumberyard as being "good behavior" on his part.
When seeking help at the Sodor Ironworks, Aura often seeks to borrow Diesel 10 based upon his gruff personality and hate for steam engines. As such, he was seen lurking about the Ironworks during the events of "Grim Messengers of Doom".
Diesel 10 has worked alongside the duo when he is called to the Ironworks.
'Arry and Bert seem to enjoy 10's company, as they ask him if he will come by to the Ironworks after their meeting with the Lumberjacks. Diesel 10 darkly assures them he will, on the condition that there is plenty of "fresh and soon to be lifeless metal that needs to be torn apart while simultaneously begging for mercy."
During the events of "Rock-Star", the twins are the first engines the diesel encounters when he reaches the blockage. He greets them, gloating about his temporary return to the North Western Railway. While 10 may seem to to be an intimidating force, Bill and Ben are able to poke holes in this tough customer. For instance, when Diesel 10 insists he is not built for moving rock, they remind him that they have seen his "artwork" in multiple places on the island. He retorts this by claiming it was only a passing artistic phase, but nonetheless undermining his credibility.
Diesel 10 recently overheard Derek talking with himself at the Lumberyard. He has become interested in the Derek's supposed disenchantment with steam engines, sensing a possible alliance with the conflicted diesel.