Lumpy is like a guardian to the Happy Tree Friends, as he is one of the main adult characters on the show and holds the most professions. It is thus safe to say that he knows a lot of the characters. There have been episodes that have made Lumpy out as selfish, such as Wingin' It, where he takes all the parachutes. But overall, Lumpy is a well-liked character. However, his stupidity often gets in his and others' way.
- Main article: Cuddles-Lumpy Relationship
Lumpy is shown to be a good friend of Cuddles. He has brought Cuddles, Giggles, and Toothy to play in the park, as seen in Spin Fun Knowin' Ya and Blast from the Past. In Hear Today, Gone Tomorrow, he plays with Cuddles and even wants to celebrate his birthday, baking a carrot cake for him.
- Main article: Giggles-Lumpy Relationship
Lumpy and Giggles are great friends that sometimes have quarrels. In I've Got You Under My Skin, Lumpy and Sniffles save Giggles' life and heal her. In Every Litter Bit Hurts, however, Giggles is angry with Lumpy for continously harming the environment.
Lumpy and Toothy are seen as friends most of the time, but they still do have problems. In Strain Kringle, Toothy throws a snowball at Lumpy, causing him to throw a rock. In We're Scrooged!, Lumpy kills Toothy for his body parts.
Lumpy has been shown to be friends with Petunia. In You're Bakin' Me Crazy, he attends her birthday party. In Wishy Washy, he does her plumbing. He also makes a movie with her in YouTube Copyright School and they later peform the movie live.
- Main article: Lumpy-Handy Relationship
- Main article: Sniffles-Lumpy Relationship
Sniffles and Lumpy can vary between like and hate in the series. While they can be friends at some points, there are times in which the two share a conflict, mostly due to their opposite IQ scores. In Happy Trails Pt. 2: Jumping the Shark, Sniffles, Lumpy, and several other characters work together to escape the island they are stranded on. However, this comes to an end when Lumpy is thrown off the rocket ship by the other passengers.
Nutty is one of Lumpy's scouts in Take a Hike. He is also a patient of Lumpy's in Chew Said a Mouthful, but this does not end well as Nutty kidnaps another patient, Toothy. Lumpy makes a failed attempt to save Nutty in All Work and No Play. The two are seen playing together in Milk Pong. On the negative side, Nutty attempts to shoplift in Icy You when Lumpy has his tongue stuck in a hot dog roller. One of Nutty's fantasies in A Sucker for Love features the implication of him murdering Lumpy over a box of chocolate.
Lumpy and Disco Bear do not interact much. However, in A Change of Heart, Lumpy tries to help him find a new heart, but only manages to get one from a whale. In Ipso Fatso, Disco Bear gives Lumpy money when the latter is hurt. They are also police partners in A Vicious Cycle.
Lumpy does not interact with Pop very often. In Stealing the Spotlight, Lumpy is shown to envy them as they decorate their home, prompting him to do the same to his home as a challenge. In Spare Tire, Lumpy throws away Pop's smoking pipe. They do get along in a few episodes, such as A Vicious Cycle, where Pop invites Lumpy and Disco Bear into his home.
Lumpy does not interact with Cub very often either. However, in Read 'em and Weep, he cures Cub of a demon, and is seen sad about his death.
Flaky and Lumpy are often shown to be good friends. She is included in groups Lumpy lead in From A to Zoo and Take a Hike. In the former episode, he cheers her up by making her a balloon animal and then carries her back to the bus. In the latter, he is horrified seeing Flaky's corpse as a bird nest. Lumpy also nurses Flaky back to health in Party Animal, swiping away Mime's peanuts because of her allergies. In Out of Sight, Out of Mime, which is one of the few times where Lumpy and Flaky interact as friends without Lumpy having more authority over her, they go trick-or-treating together (along with Giggles). On an unfriendly note, Lumpy kills Flaky in Happy Trails Pt. 2: Jumping the Shark for puncturing a raft.
Mime and Lumpy do not interact frequently, but when they do Lumpy usually shows hostility to Mime. They are shown to be friends in Home Is Where the Hurt Is, as Lumpy is being entertained with Mime's tricks. However, they are not on such good terms in Mime to Five, where Lumpy makes Mime clean elephant droppings after being unimpressed by his circus tricks. In Happy Trails Pt. 1, Lumpy yells at Mime while he's choking and in Party Animal, he swipes away Mime's peanuts.
- Main article: Lumpy-The Mole Relationship
Lumpy and The Mole are usually friends. The two are seen alongside each other in episodes such as A Hole Lotta Love and Concrete Solution. Lumpy also gives The Mole a seeing-eye dog in A Sight for Sore Eyes. Sometimes these interactions are not so friendly, like in Don't Yank My Chain.
- Main article: Lumpy-Russell Relationship
Lumpy and Russell are shown to be good friends on a number of occasions. In Get Whale Soon, Russell and Lumpy try working together to escape the whale's belly. In Sea What I Found, they are both content with finding and sharing sunken treasure. They both an enjoy a trip together in Snow Place to Go and they are both in a band in In a Jam.
- Main article: Lumpy-Lifty-Shifty Relationship
These are some of the few characters that Lumpy dislikes, as is shown in episodes like Meat Me for Lunch, Milkin' It, and Junk in the Trunk. He may still care about them though, as is seen in The Wrong Side of the Tracks and Happy Trails Pt. 2: Jumping the Shark.
- Main article: Lumpy-Flippy Relationship
Lumpy and Flippy's relationship can be considered neutral. Lumpy sees him as a friend, as is shown in By The Seat Of Your Pants. But like all characters, Lumpy is terrified of Flippy's evil side and will not hesitate to kill him if the chance comes by.
Lumpy seems to like Splendid, though they do not interact often. In It's a Snap, Splendid rescues Lumpy. In See What Develops, Splendid is employed by Lumpy. In Breaking Wind, he saves Lumpy again but cares more about his book.
Lumpy and Cro-Marmot do not seem to be very fond of each other. Their main interaction is in Wipe Out!, where they are rivals in the surfing competition. Lumpy mocks Cro-Marmot for most of the episode whilst showing off his surfing skills, but in the end, Cro-Marmot ends up on top, not caring about Lumpy's fate. They can be friends at some points however, as In a Jam seems to show Lumpy being impressed by Cro-Marmot's guitaring skills, and Cro-Marmot seems to get paid a large amount of money by him in Mime to Five.
- Main article: Lumpy-Lammy Relationship
Lammy is another character that is not regularly on good terms with Lumpy. In A Bit of a Pickle, Lammy tries to explain Flaky's death, but Lumpy tazes her in the eye. Lumpy later kills her intentionally in All In Vein, along with her friends. He does seem to care about her in All Work and No Play, though he does very little to help with her very serious injury.
The closest thing to an interaction Lumpy has had with Mr. Pickels so far occurred in All In Vein, where he kidnapped and ate him off-screen.