The MC is a future Avatar who got swept up by the spirits and thrown into a big spirit mission that not even he understands the objective of. It's a mystery. Throughout the game there seems to be some hints possibly alluding to what's coming, but nothing concrete. Book 5, the epilogue is supposed to finally give some answers and bring everything to a close. It's only a matter of time now before it releases.
And a lot of folks here are sweating it, all worried the ending is going to disappoint or something. Those people have gotta relax. We enjoyed the ride, didn't we? We had a pretty good time as this went with a lot of laughs and a lot of feels. Especially in books 2 and 3. In the grand scheme of things I don't really care how it ends. If we get something amazing then great. If not then I still got quite a good amount of quality content out of these first 4 books.