Jack Henry was the son of Jim Henry, sometimes known as The Last Cavalier. For many centuries following the Great Dying, the world was in a state of peaceful anarchy. The few remaining humans were either nomads or farmers. As the population grew, a need eventually arose for some sort of social order. The Cavaliers became that force. They were police, judge, jury and sometimes even executioner tasked with keeping people civil to one another. Twice a year, they would go on what was called "The Long Ride" where they would circle around their assigned geographical jurisdiction. There were two Cavaliers assigned to each area and when their paths crossed, they conferred and compared notes. Being a Cavalier was a noble calling and the mantle of responsibility was handed down from father to son. Jack Henry was groomed to be his father's successor.
Jim Henry was the first to discover the Ark Lords. Jim was captured and killed by the Erklirte. His wife, Helen, was also captured by the Ark Lords and enslaved. Jim and Helen's son, Jack, was left to his own devices. It took him many years to organize a militia and they began a guerilla war against the Erklirte. Eventually, Helen escaped and told the now-adult Jack what the Ark Lords were after. Jack put together a band of fifty fighting men and they took a stand at what later became known as the Battle of Chicago. Many, many good men died that day but whatever the Ark Lords were after, they were stopped before they could find it.
Jack Henry, later called Hanry Ta Jihn in the Vuduri language, died a martyr. His stirring call to action mobilized the remaining masses and though it took many, many deaths, eventually, the Ark Lords were defeated. Hanry Ta Jihn's dying request was that after the Erklirte were vanquished, all evidence of the Essessoni culture be "erased." It took several centuries but eventually, the people who became known as the Vuduri acquired enough technology to grant Hanry Ta Jihn's dying wish.
By the time Rei got back to Earth, there was no evidence that his people or the Essessoni culture ever existed beyond the artifacts of the Ark Lords which were preserved for all time as a warning that the Earth must be ever vigilant against the return of the Erklirte.
We will learn more about Jack Henry - Hanry Ta Jihn - in the novel entitled The Ark Lords.