Since his rebellion at Tuanul at the very beginning of The Force Awakens, Finn has been posited as the center and in many ways focal point of the conflict between the Light and the Dark.
Not only is he the character that connects Rey and Poe, the two other members of the trio who have no interaction at all until the very last minutes of TLJ and even then their interactions makes it clear that their connecting point and relation to one another is Finn.
What is more he’s commands the interest of all the three main villains, not just Phasma and Hux who would have a natural interest in him, but Kylo is near obsessed with him in TFA. This is unlike any of the other trio members, Kylo’s interest in Poe is only for information and Hux and Phasma has no interest at all in Rey.
This not only makes Finn the bridge between the Jedi (Rey) and the Resistance (Poe) on the side of the Light, but also puts his squarely as the focal point of the conflict with the Dark side.
This position as the center of both military and Force related conflict is reinforced in TLJ at several points.
As previous mentioned he remains throughout the movie the bridge between the Jedi and the Resistance. He is the one who, when Leia is incapacitated, spares a thought as to how this will affect Rey and by extension the possibility of the Jedi being brought back.
His choice in this hopeless situation that the Resistance finds itself in, is to take the guiding beacon away so that Rey won’t run into an ambush when she returns. When he realizes that all is not lost he gives the beacon to Poe, entrusting him with that future as Finn himself once again runs off into danger.
Another point is in Luke’s words to Kylo where the movie cuts to each of our trio in turn, one for each line.
“The Rebellion is reborn today”, cut to Poe.
“The war is just beginning”, cut to Finn.
“And I will not be the last Jedi”m cut to Rey.
Again the story positions Finn in the middle of the conflict, a conflict that involves both a “mundane” military side and a Force side.
With all of this two points in TLJ become very interesting indeed.
The first is that Rey took the Jedi texts from the Force tree and it is Finn who discovers them in a drawer on board the Falcon.
The other is that when Anakin’s lightsaber broke so did the kyber crystal inside it, making a single legacy now divided into two parts indicating two future wielders.
One we all expect will go to Rey. But with his position so squarely being set up to be both military and Force related, I think the second part belongs to Finn.