I missed something there, Could be someone Roger Pirates met in the way. Shanks call him old friend and not old crewmate may be because the person didn't voyage together. Shanks wasn't a newborn in chapter 0, they were invited by Shiki not once.
Just speculation, if Whitebeard was very close to Roger. There could be a chance Shiki also was very close to Roger and possibly their crew. I never said he is the man marked by flame. He could but didn't, I wish I have the answer.
I am leaning more towards the silhouette being a new character.