In Stranger Things Season 4, it was revealed that the mastermind behind every event in the series was caused by one man: Henry Creel. Henry was a new character in Season 4, introduced as an orderly assisting Dr. Martin Brenner at the Hawkins National Lab. As Eleven uncovers traumatic memories of her past in the Lab, she learns that Creel is not only the supernatural ruler of the Upside Down, but was the first test subject in the Lab.
Source：https://www.cbr.com/who-is-001-stranger-things/
