In the film, he has a muscular physique, light olive-green skin and wears a black cowboy hat, red bandana covering his mouth, a skull belt, sunglasses, a knife necklace around his neck, leather black pants, light gray shoulder pads, and a black ripped up vest, and 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' bandana lacks the white spots, and his shirt is changed from gray to white in the game.
He was a mercenary who is hired by Sheldon J. Plankton to kill SpongeBob SquarePants and Patrick Star. The reason for this is 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 is retrieved within a period of six days; Plankton, obviously, wants Krabs to die from a painful and wrongful execution, and upon learning about 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 him, Dennis 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, who begins to tell him "All bubble-blowing babies will be beaten senseless by-," but before he can finish, Dennis beats him up, sending him flying to the bar.
Dennis then drives away and moves on. When he arrives at the skeleton field, he begins laughing upon crushing a skull. Sometime later, he arrives at the other side of the trench and waits 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 men with the seaweed mustaches that Princess Mindy gave them earlier. Dennis then rips them off and reveals to them that they are fake. He then shows them his real mustache.
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 had 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 back to Bikini Bottom on the backside of David Hasselhoff. 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 walks up to SpongeBob and Patrick and the latter suggests that they fight back against him.
Dennis quickly swats him aside and goes after SpongeBob, who tries to bribe Dennis with Goofy Goobermoney and bubbles, which damage Dennis's eyes. Irritated, Dennis prepares to finish them but is knocked off of Hasselhoff by a passing boat.
Dennis appears as the boss in two of the four boss levels in the movie game adaptation: "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, who counters and defeats him. 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 attack to defeat Dennis. After being defeated, he is about to approach SpongeBob, though he still gets hit by a boat.
Dennis appears twice and is defeated by pure luck both times. First, he is stepped on by the Cyclops and then is hit by a boat.
Dennis is one of the only characters in the entirety of the SpongeBob SquarePants universe who is a hitman.
Patrick thinks he is a mermaid after he shows him and SpongeBob his mustache.
After Dennis gets crushed by the Cyclops, his bandana disappears and he is never seen with it on for the rest of the movie.
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 is underneath "bigger boot," he is crushed very much like Plankton. There are even dark green wrinkles on him that resemble Plankton's unibrow, eye, and mouth.