In "Stranger Things," the human birth name of the villain who becomes Vecna is Henry Creel.
Henry is the son of Victor and Virginia Creel and the older brother of Alice. As a child, he develops powerful telekinetic and telepathic abilities and becomes increasingly alienated from ordinary human life. His family later moves to the Creel House in Hawkins, where Henry uses his powers to murder his mother and sister and frame his father.
Dr. Martin Brenner takes Henry into government custody and turns him into the first numbered test subject, One. Years later, Henry works as an orderly at Hawkins Laboratory and manipulates Eleven into removing the device that suppresses his abilities. He then massacres the other children in the lab.
Eleven defeats Henry and sends him into the Upside Down. There, severe injuries and long exposure transform him into the scarred entity known as Vecna. Thus, Henry Creel, One, and Vecna are different identities or stages of the same character—not separate villains. Victor Creel is Henry’s father, which is the source of a frequent mix-up.