Who is Isaac from The Descent: A Novel and what is their importance?

Isaac, once a pilot for the British air force, was apparently shot down during World War II and taken captive by the hadals. He has since become a powerful member of the hadal community, which gives him the right to roam anywhere he likes under the earth. Isaac tricks Ike's mountaineers into believing they're safe, and he then kills the majority of the group when he lures them deeper into the earth.