On Stranger Things, young heroine Eleven's origins are murky at best and the series divulges precious few hints about where she came from. She doesn't seem to know anything about the world outside of the hospital that haunts her in flashbacks. Her closest relationship is with the creepy Dr. Brenner, who overseers the experiments Eleven acts as a guinea pig for. She even refers to him as "Papa." Dr. Brenner treats her more like an asset than a child, but there does appear to be some affection between them, as twisted as it is. But is it possible that he's her real father? Is Eleven Dr. Brenner's daughter on Stranger Things?
The fact that she calls him Papa would indicate that he was, but it could just be a manipulation on the doctor's part to make her connected to and loyal to him. There is an implication on the series that Eleven's mother might be a woman named Terry Ives who also worked with Dr. Brenner years ago as a test subject. She hadn't known she was pregnant when she was undergoing the experiments that apparently led to her baby having special powers of telepathy and telekinesis. If Eleven is her child, it would explain where Eleven's abilities came from.
There's no mention made of the baby's father, but Dr. Brenner is a definite possibility. Terry and Dr. Brenner knew each other right around the time her daughter was conceived, and it's plausible that they were involved and that he got her pregnant. If she is indeed Eleven's mother, then Dr. Brenner could be her real father.
If Dr. Brenner was Eleven's father, it would explain a lot. He is extremely possessive of Eleven, going to any lengths to get her back; this could be motivated not only by the fact that she's his successful experiment and secret weapon, but because she is his child and he feels he has complete control over her. He can also be affectionate towards Eleven in a way that feels too familiar for a doctor and patient. He holds and cuddles her like a father would a daughter, but this could just be another tool of manipulation.
Despite that, their bond is a deep one. The only picture Eleven keeps on her wall in her bedroom at Hawkins Lab is a drawing she did of herself and Dr. Brenner. He's the only person Eleven has in the world until she escapes and finds new friends in the neighborhood boys. Even if he's not her biological father, a fatherly role is still the one he plays in her life, though it's distorted by his ulterior motives.
Whether he feels he has genuine love for her or not, he still uses her for his own purposes to the detriment of her mental and physical health. And that is not something a father should do.