User blog comment:Vremyanova/Speculated Future Servant, Short Introduction, and Their Possible Class/@comment-24050152-20160129140700/@comment-27654842-20160129141646

I rewatched the opening and I actually saw a figure of young boy with white hair and pale skin standing in Chaldea's hallway near the ending to the opening.

The first person who came in mind is Sieg, the homunculus from Fate/Apocrypha, but I could be wrong.