Mr. Pickels is one of the main characters of Happy Tree Friends. He is Lammy's debatably imaginary (villainous) friend, and they are never seen separated. He is a pickle and the only non-animal main character in the show. Like Lammy, he does not appear in the TV series, as he debuted in 2010, whereas the TV series aired (and ended) in 2006.
As seen in his debut episode, A Bit of a Pickle, and his next starring role, Royal Flush, Mr. Pickels is one of the characters who kills the other Happy Tree Friends on purpose, though whether or not he actually does this is debatable as he may or may not be a figment of Lammy's imagination. He seems to like tea parties, and was seen having one with Lammy in the beginning of A Bit of a Pickle.
Mr. Pickels (in Lammy's imagination possibly, as he often appears as a normal pickle) wears a top hat and has a moustache. Many fans believe that Mr. Pickels is the representation of a repressed dark half of Lammy, similar to Flippy's "flipped-out" state.
Although he is similar to Flippy, the difference is that Mr. Pickels presents a new opportunity for the show to set up unique scenarios where one character is blamed for another's actions, leading to some awkward results.
If he is real, Mr. Pickels could also be jealous of other people around Lammy and afraid of being replaced by them, thus he tries to kill them, since he would want all of her attention to himself rather than let her have real friends. After he kills someone, he goes back to his normal pickle form, which gets Lammy framed.
He is also an antagonist of the series, murdering people (such as Petunia and Flaky) regularly and on purpose. He is one of the most violent characters in the series along with Fliqpy and The Ants.
For now, Mr. Pickels is the only non-animal character in the show. It is possible that more of his kind may appear in the future, especially considering Lammy's possible schizophrenia.
If Mr. Pickels is a living creature, he and Lammy seem to have a relationship similar to that of Lifty and Shifty in that they are usually shown to be very close and never seen without each other, but Mr. Pickels always frames Lammy for his murders (or at least never reveals himself when she's blamed) and Lammy never even hesitates to throw the blame on him. This shows that despite their love for each other, when it comes down to it, they both care more about themselves than each other.
All In Vein: Gets his head bitten off by Lumpy. (Since he might just be an inanimate object)
Breaking Wind: Is either killed by Splendid's gas or by Petunia setting the world on fire. (Assuming he died at all, he likely died from the latter since, being a pickle, he may not need oxygen to breathe)