Nice. Unfortunately, that doesn't explain the tapes where we hear him speaking or the fact that the guards talk about him. Perhaps he is real but still fulfils that kind of role?
To quote myself:
Empty Cassette tape/PTSD influenced hallucinations, like in
. His subconscious is being projected into the world around him.
Big Boss has been through so much shit in his life that it's totally possible that he could be suffering from some serious PTSD.
Also, Skull Face is described as "A ghost without a past" (because he is the past) and he has been described as "somewhat inhuman," especially when Kojima said "he’s not just a plain old bad guy: his face has been stripped off, revealing his skull. This ‘skull face’ represents the human side that he lost."
To note a cassette tap from Ground Zeroes:
Skull Face said:
I needed this place, to listen to the words people have to say. That's something they could never understand. (the man, Skull Face, then walks up to the prisoner, who is groaning in agony) You've done your part. You told us plenty. You've known the pain of ages. And even now you think, as any person would, that this can't be happening. Is it education? Morals, faith? Just an imprint of a lifetime of stories? Face to face with oblivion, which is where you are, and you still think that help is coming. The world you were born into is made to save you. Isn't that right? Of course it is. Everyone knows that. Until your last breath, you know it. Without the slightest chance or reason left to them, humans are capable of hope. I'm no different. But for one thing. When my time came calling I didn't die. My family died, my country died, but they didn't take me with them. All Hell took from me was this skin, this outer peel that marked me "human."
Here, this is Snake's subconscious reflection upon his own world views, his desperate hope that in the world he lives in, things will turn out to be better, that he will be more than just a weapon in his life. He also talks about what has been stripped away from him, such as The Boss (his family), America (his country), and his values/worth (his skin), and basically says he has nothing left.
Skull Face said:
My village had an oilseed field and a fine factory. Every day my friends and I would see our parents at work in that factory. That's all I had. All the world I knew. Then one day, aircraft came droning in from some far-off sky. The factory was bombed. Some... "spies" had told them we were making weapons. The building burned. We tried to flee outside. The crowd blocked the exit. The crowd of people. Hot. So hot. I tried to push through their legs and get ahead, but a boot in my stomach put me on the ground. The smoke of them burning filled me up. I heard my name called... but not for long. At the infirmary they carried me to, a nurse in the corner saw me and remarked, as if it happened every day: "They should let the poor thing die." Those are the only words of my mother tongue I remember. It was the language of my village. Until foreign troops invaded. Then the last identity I had left - the words I spoke - were pulled from me. My skin would never feel anything again. This face would be burned again, in torture, at foreign hands, but I, I still writhe in that burning factory. Doused in scalding rapeseed oil. That's all I have to feel, that pain - all I have to remind me I exist here. (chuckles) Those "spies" reported well. We made weapons all right. As cartloads of rifles came in from the battlefields, we fixed them up and sent them back out. So our country could win. Or rather, so that little world we knew could continue. I came to realize I mustn't die. I'm their last hope, all those who perished and left me here. I have to accomplish something. If I don't, their will will be swept out of this world.
Basically, this is an allegory for Snake's past that can tie into the quote above. His village was him, the factory was America, and the bombers were his betrayal. The rest is basically just saying his beliefs turned on him. The rifles he talks about were the soldiers he recruited to MSF.
Skull Face said:
(Skull Face proceeds to grab the prisoner) So? Do you see me now? Tell me. What do you see? Hmm? You have eyes. What do your eyes see? (chuckles) Yes, that's right. You see a skull face. You see me. This skull is who I am. My mark, my proof of humanity. I have no country, no language, I have no face, but I haven't lost my skull. So I told myself... The pain and effort that keep me alive will never know relief, never bear fruit, never be repaid. I know that, but I told myself to focus on some hope, a nonexistent hope to guide me through this burning world. A hope - call it a dream. A melancholic delusion. As the pressures within me stretch me to bursting, and I force myself not to cry out, though the words I thought were carved into me are gone, and all I knew is dead. I know how you feel. I've felt that. So show me. That I'm not the only one. That you too can return to this world, for revenge. Do you see me? Don't die. Don't die! (Ahh...)
This is pretty much just Snake's subconscious telling him to give into the pain and understand what the world really is, which is what led him to create Outer Heaven.
TL;DR: I think a lot of things about Skull Face's backstory can aline with aspects of Snake's life.
Anyways, maybe Skull Face is a real character, but this could be what he represents in the story of MGSV.