Stranger Things: Who Is The American?
7. Billy Hargrove
This might be the most unlikely of the options, but it's still not impossible, Billy Hargrove, AKA your girlfriend' s favorite character came to a heartbreaking demise after we learned his whole backstory with an abusive father and his beloved mother leaving the family as a result.
Billy is very unlikely to be the captured American simply because, well, we all watched him die, pretty gruesomely too, sacrificing himself to save Eleven after tapping into his "source".
Or did we?
Before Billy became infected by the Mindflayer, he was in the Upside Down staring at another version of himself. It’s entirely possible that this version is the one causing all of the terror in Hawkins while the real Billy is still trapped in the Upside Down. Besides, in the "three months later" closing scene, we never really saw any semblance of a funeral, or mourning.
We saw Max looking upset staring at what was presumably Billy's bed, but we didn’t see anything like a tombstone, or even his name in the "Cutting Edge" clip, where it mentioned all "mysterious" deaths including Hopper, Bob, and Barb.
So again, it's very unlikely the real Billy was captured by Russians in the Upside Down, but that doesn't mean he isn't still alive.