In the film, he has a muscular physique, light olive-green skin, a black cowboy hat, red bandana covering his mouth, a skull belt, sunglasses, and a knife necklace around his neck. His pants are leather black. He wears light gray shoulder pads, and a black ripped up vest. He has black biker boots on his feet, which have gray, metal spikes hidden underneath. On his right hand-shaped fin, he wears a black, fingerless glove that trails a few inches up the wrist. This glove is covered in similar, yet less sharp spikes. Under his right shoulder pad is a gray flap, overlapping a darker gray flap.
In the game, Dennis retains his overall appearance, but he is now depicted with a black mohawk underneath his hat, and with scars on both sides of his face. Also, Dennis's bandana lacks the white spots, and his shirt is changed from gray to white in the game.
He was a mercenary who was hired by Sheldon J. Plankton to kill SpongeBob SquarePants and Patrick Star. The reason being that Plankton stole King Neptune's crown and tricked Neptune into believing the crime was committed by Eugene H. Krabs, so Neptune threatened to execute Krabs (via burning by fire) unless the crown was retrieved within a period of six days; Plankton, obviously, wanted Krabs to die from a painful and wrongful execution, and upon learning SpongeBob and Patrick's journey to get the crown back, he sends Dennis after them.
During the journey, Dennis encounters the gas station that SpongeBob and Patrick had gone past earlier. When Floyd and Lloyd begin insulting Dennis by asking him "Does that hat take ten gallons?", Dennis does not like this at all and rips off their mouths before moving on. Later, as SpongeBob and Patrick are being chased by the frogfish, Dennis stops at the thug tug. He blows a bubble and finds an image of Patrick and SpongeBob inside to track them. He is then confronted by Victor whom begins to tell him "All bubble blowing babies will be beaten senseless by-." Dennis also did not like that at all and simply punched Victor, sending him flying to the bar. Dennis then drove away and moved on. When he arrived at the skeleton field, he began laughing upon crushing a skull. Sometime later, he arrived at the other side of the trench and waited to ambush them on the other side.
SpongeBob and Patrick encounter Dennis for the first time after reaching the end of the Deep Dark Trench. Dennis explains to them that he is a hitman hired to exterminate them. SpongeBob and Patrick laugh him off, believing that they are invincible with the seaweed mustaches Mindy gave them earlier. Dennis then ripped them off and revealed to them that they were fake. He then showed them his real mustache, causing Patrick to wonder if he was a mermaid.
As SpongeBob and Patrick tremble with fear Dennis tells them that he wants to step on them so they never know that Plankton stole the crown. Having enough chatting, Dennis prepares to kill them with his "big" spiky boot, only to be stepped on by the Cyclops' (a scuba diver) "bigger boot." In the climax of the film, SpongeBob and Patrick hitch a ride on the backside of David Hasselhoff, who speedily swims through the ocean in order to send them back to Bikini Bottom. During the middle of their swim, SpongeBob and Patrick see the boot belonging to the same diver who squished Dennis. Dennis then detaches from the boot and onto Hasselhoff, revealing that he is still alive. He walked up to SpongeBob and Patrick and Patrick suggested that they fight back against him.
Dennis quickly swatted him aside and went after SpongeBob and eventually grabbed him after a chase around Hasselhoff. SpongeBob tried to bribe Dennis with Gooby Goober money and with bubbles, which damaged Dennis's eyes. Irritated, Dennis prepared to finish them but is knocked out of Hasselhoff by a passing boat, presumably killing him as he is never seen again.
Dennis appears as the boss in 2 of the 4 boss levels in the Movie game; "Name's Dennis" and "Dennis Strikes Back!"
In "Name's Dennis," he places SpongeBob on a stick and attempts to throw projectiles and use smaller enemies to defeat Patrick. Patrick counters and defeats him, and as Dennis is about to counterattack them, he is stepped on by the cyclops.
He, deformed physically, later appears in "Dennis Strikes Back!"; this time, SpongeBob fights him and he must use the bowling ball (and if possible the reflective macho strike attack) to defeat Dennis. After being defeated, he is about to approach SpongeBob, though he still gets hit by a boat.
Dennis appeared twice and was defeated by pure luck both times. First, he was stepped on by the Cyclops and then was hit by a boat.
SpongeBob and Patrick were most likely the first people to successfully escape Dennis due to the fact that he quickly went insane after being defeated only once. Judging by how Dennis claimed that he "always gets his man," it sounded like nobody prior to SpongeBob and Patrick has ever escaped him before and he succeeded into killing his targets.
He looks different in the movie game. He has a white shirt and blue pants. However, when the player fights him later in the game, his shirt is gray and his pants are black. It is unknown why that happened. (Only in-game, in the stills from the movie cutscenes, they are still white and blue.)
Dennis is one of the only characters in the entirety of the SpongeBob SquarePants-universe who is a hitman.
After Dennis gets crushed by the Cyclops, his bandana disappears and he is never seen with it on for the rest of the movie. It is possible that it fell or got ripped off.
In the French dub of the movie, Dennis was voiced by Greek-French TV host Nikos Aliagas.
In his last scene of the movie when he is about to kill SpongeBob and Patrick one last time and telling them that he will "see them later," the sheen on his boots are gone.
While he was underneath "bigger boot," he was crushed very much like Plankton. There were even dark green wrinkles on him that resembled Plankton's unibrow, eye, and mouth.
Dennis is one of the deadliest villains in the franchise. He ripped Floyd and Lloyd's mouths off, punched Victor out of his socks and into the Thug Tug, presumably killing him, and tries to kill SpongeBob and Patrick twice, but failed because of him getting defeated by luck.