I think he is the founder of hawkins lab

Dr.Brenner is not her biological father,nor is Hopper.Its Terry‘s boyfriend Andrew(i dont know the surname)

No one important

There is one problem with Andrew being ELS father,

Ya see, eleven was born in November 1971 and since a woman has a 9 month pregnancy, El had to be conceived in February of 1971

But andrew passed away sum time in 1970, so that makes it impossible for Andrew to be els father

Yeah but in the origin story of Hawkins Lab she said it was Andrew's baby.

It's Andrew Rich, he was introduced in the Stranger Things novel Suspicious Minds.

Who is Andrew

He was Terry's boyfriend in College and one candidate for being Eleven's birth father. However he was drafted into Vietnam and it is rumored that he is dead in Suspicious Minds.

Well there's one crazy Theory on Reddit that I heard that Eleven was actually Sara Hopper (Hopper's Daughter) and the girl who died was a fake kid to replace Eleven as Sara Hopper. This is totally not true unless Hopper had an affair with Terry Ives, which this being discovered could actually be the reason why Hopper's wife ditched him.

I think this would be crazy....but... what if Eleven's father would be...The Mind Flayer?