Eleven’s adoptive father in Stranger Things is Jim Hopper.
Hopper is the chief of police in Hawkins, Indiana, when the series begins. He first becomes involved in the search for Will Byers and gradually discovers the connection between Hawkins Laboratory and the alternate dimension.
After finding Eleven, Hopper protects her from government agents and eventually becomes her legal adoptive father. Eleven calls him “Papa” at times, a term also associated with Dr. Martin Brenner, so the two father figures are distinct and should not be confused.
Hopper is played by David Harbour. His relationship with Eleven is one of the series’ major emotional storylines, especially during season 3 and the events that follow the Battle of Starcourt.