The helmets appear as a large oval, covering the head just above the nose, the sides cover the ears completely. There is a visor on the helmet that apparently allows the user to see from it. There are also rectangular holes to the side, likely to allow the user to hear when wearing the machine.
During the time Misaki was in the Exterior Project, the Mental Guards were sufficient enough to protect the researchers from her ability. However, they state that if Misaki continued to grow the Mental Guards will eventually be not enough to protect them.