As it is seen in many episodes, Flippy and Lumpy are neighbors, and the two get on well most of the time when appearing together. The only problem with their friendship is that Fliqpy always tries to kill Lumpy (with a little success due to Lumpy's extreme luck).
Lumpy seems to be friends with Flippy on multiple occasions. They are shown to be neighbors and he invites Flippy into his house in By The Seat Of Your Pants, in Happy Trails Pt. 2: Jumping the Shark he seems pleased to see Flippy hammer down Flaky's grave for what she did to their boat, and he even attempts to help Flippy with his problem in Double Whammy Part I in every way he can. He also calls for help when Flippy and Cuddles are killed in Hear Today, Gone Tomorrow.
He does not seem to hold any obvious issues with Flippy aside from a certain level of apathy in blasting his speakers loudly so he can hear the TV in Hear Today, Gone Tomorrow. However, this could be attributed to stupidity. Lumpy does the same thing in In Over Your Hedge with his lawn machinery, but he apologizes whenever Flippy tells him to keep it down.
His relationship with Fliqpy is of course very different though. Whenever Flippy flips out, Lumpy is very quick to defend himself, usually with great success, either by flipping him back like in Double Whammy Part I or by killing him like in Remains to be Seen and By The Seat Of Your Pants.
Flippy likewise seems to be quite fond of Lumpy. He visits Lumpy's trailer to watch TV and eat cheese and goes to Lumpy's swim race, presumably to cheer him on, in By The Seat Of Your Pants, and he trusts his psychiatric advice in Double Whammy Part I. Despite being good friends, Flippy has his moments of showing irritation to Lumpy. The biggest example of this is in In Over Your Hedge where Lumpy annoys him with the sounds of his lawn machinery.
The only mark against their relationship on his side is when he, along with Handy and Sniffles, have no reservations throwing Lumpy out of the rocket to remove excess weight in Happy Trails Pt. 2: Jumping the Shark
As is to be expected from his character, Fliqpy wants to kill Lumpy, like he does with every other character in the show. However, he has only successfully done so in Keepin' it Reel, A Vicious Cycle, and In Over Your Hedge. Possibly because of his repeated failures, Fliqpy seems to try to kill Lumpy more obsessively than he does with the other characters, and goes to great lengths to do, most obviously in Hear Today, Gone Tomorrow. He even pursues Lumpy when he is a zombie in Remains to be Seen, despite none of the other zombies seeming interested in him at all.
Surprisingly, there is one mark for their relationship in Hear Today, Gone Tomorrow. When Fliqpy tries to shred Lumpy with his helicopter, he imagines a familiar scene of his good self riding his unicorn, and cries before flipping back.
- While Flippy has killed Lumpy three times, in Keepin' it Reel, A Vicious Cycle and In Over Your Hedge, Lumpy has killed Flippy twice as many times, in Class Act, Without a Hitch, Remains to be Seen, and By The Seat Of Your Pants.
- They are the first characters that were confirmed to have killed Truffles.
- Lumpy is the only character to win a (real) fight against Fliqpy (in Remains to be Seen and again in By The Seat Of Your Pants). Flippy defeats him in Autopsy Turvy (Double Whammy Part II), but this battle turns out only to be an extended hallucination.