...was a Synth and created the Railroad.
The entire time I have been active in the forums here, everyone has been talking about Danse being a Synth and Curie being capable of getting a Synth body, but haven't said a word about Deacon's identity. I only found this out over the weekend when, on my new character, I decided to bring Deacon with me on RR quests so I could get his affinity up (before I eventually betray them to get the BoS ending). But what is interesting is that, once he reveals to you his identity...it's almost casually, nonchalant, not at all like Danse where it's a shock to everyone, including Danse himself. I suppose it does make sense that the founder of the RR is, himself, a Synth...but it does bring up questions regarding free-will, etc., in Gen-3s.
Fallout 4 certainly is channeling the spirit/ideas of "Bladerunner"/"Do Androids Dream of Electric Sheep?", with the Institute, Synth paranoia/fear, and Synths popping up left-and-right within your group of companions. Nick, Deacon, Danse, and Curie all bring a very different insight into what it means to be a Synth in the Commonwealth wasteland.