mr.pickels

As seen in his debut episode, A Bit of a Pickle, he is one of the characters, along with Flippy, to kill other Happy Tree Friends on purpose (debatable, since he's imaginery). He seems to like tea parties, and was seen having one with Lammy in the begining of A Bit of a Pickle.
Mr. Pickles (in Lammy's imagination possibly, as when he is a normal pickle, his outer appearance is like a normal pickle) wears a top hat and he has a moustache, like Mouse Ka-Boom. Many fans believe that Mr. Pickles is the representation of a repressed dark half of Lammy, similar to Flippy's "flipped out" state.
It isn't possible that he is the replacement for Flippy because even though he seemed to have gotten over his flipping out in Autopsy Turvy he was killed soon after probaly causing him to forget he was over it. This also presents a new oppurtunity to set up unique scenarios, assuming Lammy is a schizophrenic, where one character is blamed for another's actions, leading to some awkward results.
Mr. Pickles could also hate other people who are with Lammy, or wanting all of her attention for himself rather than let her have real friends. He would kill someone else, then whenever someone would look he would be a normal pickle, so Lammy would get framed, and no one would want to be around her.

He is also the antagonist of the series, who murders people (for example, Petunia). Lumpy thought Lammy murdered Flaky (due to Lumpy's stupidity), when it was really Mr. Pickels. He might even kill Flippy in upcoming episodes. He is the most violent character in the series, but Flippy is very violent, but not as violent as Mr. Pickles. Mr. P is the newest character to appear.