Karen "Casey" Clancy seemed at first to be a harmless mute young girl, but was in reality a powerful paranormal and sorceress, and apparently a host for a demonic entity known as the "Herald of the Antichrist".
Swamp Thing met her when she was about to be killed by her own father, and saved her at the last possible moment. The two befriended each other and began to travel together, until they were separated by an attack of vampires in Rosewood, Illinois.
- This version of Karen Clancy (Earth-One), including all history and corresponding appearances, was erased from existence following the collapse of the original Multiverse in the 1985–86 Crisis on Infinite Earths limited series. Even though versions of the character may have since appeared, this information does not apply to those versions.
- Casey's story may have drawn inspiration from the tale "Jenifer", published in Warren Publishing's "Creepy" #63, July 1974. The creators were Bruce Jones and Bernie Wrightson. 
- 13 Appearances of Karen Clancy (Earth-One)
- 1 Images featuring Karen Clancy (Earth-One)
- Quotations by or about Karen Clancy (Earth-One)
- Character Gallery: Karen Clancy (Earth-One)
Swamp Thing and is predominantly seen in the Swamp Thing family of titles. This character may be part of the Swamp Thing's supporting cast, a minor acquaintance, or one of his adversaries. This template will automatically categorize articles that include it into the Swamp Thing Characters category.