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 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 sent Dennis after them.
SpongeBob and Patrick encountered Dennis for the first time after reaching the end of the Deep Dark Trench. 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 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 the swim, SpongeBob and Patrick see the diver's boot 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 got knocked off by a boat Hasselhoff swam under, never to be seen again by the viewers. It is unknown what became of him after this, but it was most likely he just fell into the water.
Role in the series
Dennis returned as Plankton and Scar's dragon in the Lion King, only to be killed and eaten by the Hyenas in the final battle. He sometimes unexplainingly return in some of the later projects, lastly (for now) seen as he was about to kill Spongebob and Patrick when stopped by Ignightus. since Dennis unexplainingly surived many deaths in an unexplain mannor, he may return in the very future if a suitable project is found.