If "closest friend" and "so close you might ask if they were a couple" are accurate translations, then I'm happy that the anime was more forward with their relationship, as this was a bit of a painful "sappo and her friend" moment.
Here's hoping that any future Part 6 adaptation (which I only heard of the situation for, as I'm reading this all in order after watching the anime) would mean that people won't see the need to make Anasui into a man to keep the leads straight (as that is the prevailing and most convincing theory I've heard as to the character allegedly changing sexes between chapters).