: 001 is actually the young Henry Creel , who was not killed by his father but was taken by Brenner after murdering his own mother and sister.

: Eleven's "mentor" in the lab is revealed to be 001 , the powerful first subject of Dr. Brenner's experiments.

: Eleven defeats 001 by pushing him through a portal into the Upside Down , where the dimension’s environment transforms him into the monstrous Vecna . Why It Is Highly Rated

Who Is Vecna? The final moments of Stranger Things season 4 episode 7 reveal that Henry Creel a.k.a. Number One is the monster we' Den of Geek Chapter Seven: The Massacre at Hawkins Lab - IMDb

This twist recontextualizes the entire series by connecting the current villain to the origins of Eleven's powers.