I kind of imagine he's a new character, but it's kind of hard to tell because of that line you mentioned. It's hard to think of who he could be from any of the Big Boss games as I don't remember seeing anyone get caught in a fire or anything (though I haven't played Portable Ops). Volgin is the only one I can think of. There are other things that kind of make me think otherwise. There's the tape where Skull Face describes how he came to look the way he did if I remember right. I need to listen to that one again just because I was distracted and missed some of it. I would imagine he had some connection to FOX with the whole XOF thing.