If I were to guess, he's definitely closely related to Faerghus, or at least Dimitri as a person. knowing how you can see Dimitri's boot crushing his mask in the E3 trailer, which is what I'd imagine to be one of the key factors that brought him to his post timeskip state. Whether it be a person closely related to him, or an overall message, like he learns the truth of that side of the church or something along those lines - or maybe a misunderstanding of the church's goals, as I think the church being evil is too obvious, or maybe a misunderstanding/truth he learns from his own bloodline. I dunno, a lot of options, but whatever identity there is behind that mask seems like it'll hit Dimitri pretty damn hard, to say the least.