Dennis just might be Kevin's creepiest identity (other than The Beast, of course). He's the first identity we're introduced to in Split when he kidnaps three teenage girls. It's later revealed that he enjoys watching young girls dance naked for him, and he goes so far as to ask one of the girls he kidnaps to do so. (Most of The Horde seem to be grossed out by this behaviour as well and try to keep him from emerging when they can, even though he's a figure of authority, like Patricia.)
Like Kevin's mother, he also has OCD, especially when it comes to cleanliness. In Glass, he surprisingly is one of the identities who's convinced by Case and Dr. Staple that The Beast is evil. When Kevin is ultimately shot, Dennis is one of the final identities to emerge, making a comment to Casey that he needs to clean all the blood off his white pants.