I always got sorta queer platonic vibes for Frieren and Fern’s relationship. Like they aren’t romantic, but they are definitely very close, but also not in a purely platonic way. Fern clearly sees Frieren as a powerful educated teacher, but is also more emotionally mature than her in some ways. By the end of the first season I see them more as equals, at least emotionally. Given Frieren’s apparent aroace orientation, any closeness between the two would necessarily fall outside traditional romantic or sexual relationships, so people shipping the two are pretty much ignoring who the characters are to satisfy their own desires. Also it’s just kinda gross because they do have a master apprentice relationship, which would make any shipping pretty rapey, age difference notwithstanding.