Diesel, being a very rude, devious, and sinister diesel, is not very well-liked by his peers, especially steam engines whom he has wronged in the past, as well as a good portion of the island's other diesels. Whilst he does indeed have a good side, it does not show up very often, contributing to the negative image the other engines have of him.
Diesel possesses a bitter resentment for the steam engines on the island, with the feeling being mutual.
His interactions with the other diesels is ironically almost equally antagonistic, his various attempts to spearhead his colleagues as their leader does not go down well with them, and with many of the diesels not possessing the same vendetta that he does against the steam engines, this only sours his relations with them even more.
It can be assumed that Diesel is good friends with the two diesels, due to their troublesome natures. It can be assumed that they like causing mischief around the railway together.
In Munitions, when Arthur came into the military complex to try to warn everyone about the danger of the munitions, Diesel was furious and ordered him to leave, but nothing more could be said when the blaze started up due to Sir Frederick Aura.
In Hibernation, when Bear's growling noise woke up the other diesels, Diesel angrily told Bear to "run off into the woods" where he belonged.
Ironically Diesel's most positive interaction, during a period of time when Percy needed repairs and only Diesel could be spared, his violent ways with shunting nearly resulted in him being banished from Sodor for good, until he was called into help with rerailing a derailed Clarabel, whom was blocking his way home. The fact that Diesel actually did this without any real ounce of complaint and thus making this his first true good deed on Sodor actually motivated Clarabel to put in a good word for Diesel, which allowed him to come back to the island when needed.
Derek is a perfect foil for Diesel's foul behavior. Derek is kind and often tries to push Diesel towards a less sinister way of approaching matters, but is rarely successful.
In Munitions, Diesel showed no problem bossing Derek around at the complex. He told Norman to abandon Derek when the Munitions fire went out of control, so it is apparent Diesel would never "go the distance" to lend a buffer for him, let alone anyone.
However, as Bill and Ben's tricks begin to increase in frequency, Diesel's claims that diesels on the North Western Railway get shafted begins to become of interest to Derek. Knowing Diesel's tendency to overreact and exaggerate however, Derek is keeping his gaurd up regarding the issue.
The two do not get along very well, as Diesel, on his first trial to Sodor, tried to send Duck away in revenge for being made a fool of due to his arrogant nature after an incident with some troublesome trucks. Then, after telling lies about Henry afterwards, Diesel was sent away from Sodor, but returned sometime after to assist in the harbor work that was exhausting Duck and Percy, though this time, Diesel's behavior extended towards both engines in general. Even so, with Duck having been the first target of Diesel's slander, and him being suspicious of the latter ever since, it can be safe to say that out of all of Diesel's relationships, this one is the most antagonistic.
In the series, the two do not interact with each other very often, but when they do, sparks will fly. The first time was a brief quarrel in Rock-Star, when Duck and Oliver were boasting about their Great Western heritage, only to be scowled at by Diesel. After this, the two were not shown to cross paths until Perhaps He's Got a Corset. In the episode, after Duck told the big engines that he was a Pannier, not a podium, Diesel entered and commented that he would be much less a waste of space as a podium. He also made a rude remark to Duck when the Great Western engine failed to pronounce "revolutionary" correctly, much to Duck's anger.
Due to the fact that both diesels see themselves as impeccable leaders, the two often have falling outs, if they can even be considered companions at the very least. Norman is tactical and tries to view everything with neutral strategical planning, whereas Diesel's plans of action usually involve personal gain or selfish motives.
In Munitions, both Diesel and Norman attempted to assume control of the munitions operation, squabbling and arguing with each other. However, when a fire broke out due to the efforts of Walter Richards, the two beat a hasty retreat together.