Mr. Eff (Mr. Fuck) is one of the Doughboys that act as two of Johnny's inner voices. He is based on the mascots for a certain company (Pillsbury). Johnny admits at one point to stealing him. 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.  

Since Mr. Eff is technically an inanimate object that represents one of Johnny's most manic inner voices, maybe he would sound more unstable and uncomfortably unpredictable