Mr. Eff appears to represent Nny's more manic, homicidal side, as he encourages Nny to kill remorselessly to obtain blood to repaint the wall. However, this action also has a selfish motive, since as long as the monster remains inside the wall, it will feed more power to Mr. Eff and Psycho DoughBoy, making them more real. He hates Psycho DoughBoy and consistently fights with him on what Johnny should do with his insanity.
Eventually, when Johnny accidentally kills himself and the The Wall Monster escapes, Eff becomes cynical and angered at his failure to become real. Subsequently, along with Psycho DoughBoy, he is decapitated by The Wall Monster.