The core is the device that essentially protects he players’ brain while at the sleep. Without it, if the player died, the brain believes it and also dies. The core is the feat of programming protected by passwords, that only one self can remove it. At the first part of The Eye of Minds, Tanya tooked her core out metioning Kaine who Michael first heard of Kaine at the Golden Gate Bridge. It was also metioned when the Robot created by Kaine who took out Michael’s core out at the end of The Eye of Minds.