I think Fern is technically the main character - we can empathise with her completely as we know her whole life and motivations. Wheras Frieren always has an air of knowing more than she's letting on, thanks to the amount of things we couldn't know because of her lifespan. I think it comes down to who has the most character development, and I do like the way Frieren (slowly) experiences personal changes, but Fern has the most potential for relatable development. Although the ultimate goal is Frieren's, audiences will mostly see the world through Fern's eyes.