He was an assassin who was hired by Sheldon J. Plankton to kill SpongeBob SquarePants and Patrick Star. The reason being that Plankton stole King Neptune's crown, sold it to a forbidden land called Shell City and framed Eugene H. Krabs for the crime so Neptune threatened to execute Mr. Krabs (via burning by fire) unless the crown was to be retrieved within a period of six days; Plankton, obviously, wanted Krabs to die of a painful and wrongful execution, and upon learning SpongeBob's and Patrick's journey to get the crown back, he knew that Neptune would discover that Plankton was the thief so he sent Dennis after them.
SpongeBob and Patrick encountered Dennis for the first time during the middle of their journey. Dennis was about to kill them with his "big" spiky boot, only to be stepped on by the Cyclops' "bigger boot". After what happened at Shell City, SpongeBob and Patrick escape and 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 the swim, SpongeBob and Patrick see the boot of the Cyclops that he stepped on Dennis with. Dennis then detaches from the boot and onto Hasselhoff, revealing that he is still alive. He tries to kill SpongeBob and Patrick, but fails yet again. He was knocked out by a catamaran Hasselhoff swam under, never to be seen again by the viewers.
As The SpongeBob SquarePants Movie was originally stated to be the series finale of the show, Dennis was never seen again.
Despite being Plankton's minion, Dennis never interacted with his boss on-screen though due to that Plankton seems to have trust with Dennis into preventing Spongebob and Patrick from getting Neptune's crown and by the fact that Dennis wants to kill Spongebob and Patrick and was also aware that Plankton stole the crown hinting that they might be on good terms.
It is unknown what type of fish Dennis is.
He also can breathe outside of water without any trouble.