9. Traps the Atom
Part of what makes the Joker such a unique figure is his twisted mind. To help cure him of a terrible disease, some of the finest scientific minds in the DC Universe think to send The Atom, Dr. Ray Palmer, on a journey inside the killer’s head.
While he went with the intention of curing him, Dr. Palmer ended up being bombarded with the Joker’s memories in a painful onslaught of information. The memories send the Atom down a dark path himself and he almost loses his own mind while trapped away in the Joker’s head. Even though the character is directly in the Joker’s head, even he isn’t able to figure out the villain’s real origin and what actually happened within his past.