The Child/Man/Elder With the Lip Scar
Season 3: In the opening scene of season three, we see this character in all of his forms (as evidenced by the lip scar they each share) setting fire to the Sic Mundus headquarters. In both realities, he does Eva's bidding for her by helping slot events into place to preserve the knot, like getting the approval for the nuclear power plant to be built in the '50s. He's seen at Sic Mundus HQ filling in the final pages of the leather book that flows between Noah, Claudia, and Adam, adding the ever-popular phrase "The beginning is the end, and the end is the beginning" to it. We also learn that he helps to trigger the apocalypse from the power plant in both worlds. In terms of this dude in the origin world? Obviously, without a Jonas or Martha, their son can't exist.