Finn was something we have never seen before in Star Wars. A stormtrooper who develops a sense of right and wrong and chooses not to kill innocent people just because he’s ordered to. He leaves the First Order with the help of Poe Dameron and ends up meeting Rey on Jakku and the rest is history.
What is extremely entertaining about Finn in The Force Awakens, is just how naive he is. Being a stormtrooper all his life, he doesn’t really know how things work and is kind of seeing the world through his own eyes for the first time. He quickly forms a huge crush on Rey, but is fiercely loyal to her, to the point of risking the fate of the galaxy just to save her.
Later in the Trilogy, Finn is taught that there is a lot of gray in the world. Things aren’t just black and white with one good side and one bad. Many people get by by living in the middle and not picking a side. That’s an important lesson for this naive member of the Resistance to learn, and when we see him in The Rise of Skywalker he is much more mature and ready to take on a leadership role in the fight against the First Order.
Finn also learns he is Force-sensitive, and although he’s unable to tell Rey, during the final film, it’s safe to assume if Rey does indeed train a new order of Jedi in the future, that her friend, and maybe even more than that, will be her first student.