Flats' body is shown to be somewhat two-dimensional, hence his name.
He first appears as a cameo in the episode "Sandy's Rocket," where he was one of SpongeBob's "alien hunting" victims. He also makes another minor appearance in "Texas," where he attended Sandy's Texas party. His only major appearance, however was in the episode "The Bully." He joins Mrs. Puff's boating school and wants to "kick SpongeBob's butt." SpongeBob takes this extremely seriously, reacting as if he were going to actually kill him. He makes many attempts to reason with and/or escape him, but to no avail. He even calls Patrick to protect him, only to find out that Flats is with him and that they were community college buddies. He also tries to go to Mrs. Puff to help, who talks to Flats (much to SpongeBob's dismay) and explains to SpongeBob that "kicking his butt" means that Flats wants to become his friend (this is obviously not true).
Flats goes so far as to chase SpongeBob out of school into town, even trying to run him over with a garbage truck. However, the garbage truck slips on a banana peel and flips over, critically injuring Flats. He wakes up in the hospital and learns that SpongeBob saved his life through CPR, yet he still wants to kick his butt. Eventually, Flats corners SpongeBob in his own home, and proceeds to "kick his butt" by repeatedly hitting him. However, SpongeBob discovers that his spongy body simply absorbs the blows and does not harm him in anyway at all.
Flats continues to hit SpongeBob throughout the whole week until he finally passes out from exhaustion one morning at boating school.
Flats is a flounder, which is a species of flatfish which has both eyes on one half of its body.
Coincidentally, Thomas F. Wilson is known for playing Biff in Back to the Future.
Flats' name could be a pun on Fats Domino or Rascal Flatts.
It is unknown why Flats needed to attend a Boating School in the first place, as he was clearly seen driving the garbage truck.
Flats' catchphrase is "I'm gonna kick your butt!"
Even after being saved by SpongeBob he still wants to kick his butt.
Oddly he even would threaten his dad about kicking his butt just as he did to SpongeBob.