This article focuses on the interactions between Lumpy and the duo Lifty and Shifty. Please note that the article will focus in Lifty and Shifty's interactions with Lumpy as a duo and not individually.
Since Lifty and Shifty would do anything for money, they wouldn't think on how their actions affect other characters. Lumpy is their most frequent victim as they know about his lack of intelligence and know that he can be easily tricked. However Lumpy is one of the few characters they have ever shown friendship towards, signifying that they have an on and off friendship.
Lifty and Shifty’s Side
Though they have stolen from everyone in the show, but Flaky, Lammy, Mr. Pickels, and Splendid, due to Lumpy’s low intelligence making him an easy target, Lumpy is the most common victim of Lifty and Shifty’s thefts. Where many other characters have only been robbed by Lifty and Shifty once or twice, Lumpy has been a target a grand total of six times dating all the way back to the twin’s debut episode, Wheelin' and Dealin' where the three of them (as well as Handy) are participating in a car race but Lifty and Shifty’s cart does not have any wheels so, mere seconds before the race starts, they steal the wheels from Lumpy’s cart and put them on their own. Another famous example is in Meat Me for Lunch, with growling stomachs and an empty fridge, the duo steal large amounts of meat from Lumpy's butcher shop. They also take off with his wallet after (unintentionally) running him over with their van in Concrete Solution. They have even stolen large animals from Lumpy on two separate occasions: the first is Milkin' It, where they steel a cow from Lumpy’s farm, the second is in Junk in the Trunk where they steel Lumpy’s pet elephant. Finally, they steal the treasure chest Lumpy and Russell in Sea What I Found, having followed them to do so ever since they caught sight of the treasure map. Despite all this though, the only example of Lifty and Shifty disliking Lumpy that doesn’t have to do with stealing anything from him is back in Milkin' It where, when Lumpy dies trying to get his cow back, both twins can be seen taunting him.
Despite their villainous nature though, there are a few cases where Lifty and Shifty have had positive relationship’s with Lumpy being the only character besides each other that this can be said about even though in one of the examples, Happy Trails Pt. 2: Jumping the Shark, where Lifty and Shifty let Lumpy come off the deserted island in their inflatable raft, the same can be said about: Flaky, Handy, Sniffles, and Flippy, it’s only one example where there’s two more for Lumpy alone. In The Wrong Side of the Tracks, when Lumpy temporarily saved them from the rollercoaster, they were very happy and grateful. The final and most notable example though is that in Concrete Solution, when they first accidentally ran over Lumpy, they immediately stopped and got out of the van and tried to help Lumpy by applying CPR and monitoring his pulse until Shifty found his wallet by chance.
On the other side, Lumpy has on two separate occasions been shown to successfully get his vengeance for being stolen from. The first time was in Meat Me for Lunch, where after Shifty gets sent through a sausage maker and Lifty got sliced on a meat slicer, Lumpy proceeds to put both bodies in his display case and sell them to customers as meat, though this could have been unintentional as evidenced by Lumpy’s surprise when Petunia found an eyeball in her “hotdog”. The second time is in Junk in the Trunk, where after chasing the twins down persistently to get his elephant back, when Lifty was impaled with the pogo stick, Lumpy completely ignores Lifty’s death and even uses the pogo stick with Lifty’s corpse still on it.
There have also though been multiple occasions where Lumpy responds to Lifty and Shifty in a positive way, despite their reputations as thieves, though this could be attributed to Lumpy’s low intelligence and/or the show’s lack of continuity in that Lumpy might not know in these cases that Lifty and Shifty are thieves as he has never been stolen from and treated them well in the same episode. He even went against good judgement and let them into what can be presumed to be his house to enjoy a holiday dinner with him and many other characters in Kringle Feast. Another example is that in Happy Trails Pt. 2: Jumping the Shark, Lumpy and the others got mad at Flaky for accidentally popping the raft and causing Lifty and Shifty’s death and likely participated in intentionally killing her in retaliation. He also became visibly sad when he saw Lifty and Shifty brutally killed in Doggone It despite not having the same reaction to other deaths in the same episode. Even more so, in The Wrong Side of the Tracks, Lumpy went through a lot of effort to try to save them alone from the rollercoaster. A different but very notable and famous example is in Class Act where when Lifty and Shifty were shaking Flaky upside-down to create the snow effects and Lumpy clearly saw what they were doing and approved of it, implying that he liked their clever idea.