Nightmare BB is an overly exaggerated version of his counterpart, Balloon Boy. Like other nightmare animatronics, Nightmare BB is sinister, cruel, and has no qualms about torturing his victims. He has a tendency to mock his victims, and tease them relentlessly. He also seems to have a hint of gluttonous and bloodthirstiness to him. A good example of his voice is present in Ultimate Custom Night, when Nightmare BB defeats the player. 

(A good example of his voice is present on the previous Casting Call (link in project description).)