So through her teenage years she realizes, and becomes, her true self, a woman, who at no point during that time has any interest in boys.
She is still very enthusiastically only interested in girls, something she confirms to her friends, family, herself, and the audience at every occasion, and yet within the course of a week she is suddenly head over heels about boys; supposedly one specific one. A person who she has known for years, spent tens of thousands of hours being very close to, so much so that they know each other better than anyone. Yet we are supposed to believe that she somehow is suddenly attracted to him, as though a switch has been flipped. Something that in all that time she has never even entertained the idea of being a possibility. That's ludicrous. That much time with that person, at some point she would have definitely thought about it, if it were indeed even remotely true.
She has continued to encourage him to be with women that she knows he enjoys being with, and is genuinely sorry for him when he breaks up with the girlfriend he loves. Despite all that, and every indication to the contrary, she is just all of a sudden sexually attracted to him, and whole internal monologues are devoted to that major inexplicable shift. Seems to me it's a plot device to throw in a "twist", and is completely out of place. It doesn't fit Emma's personality, anything we know about her, the history they have shared, her prominent sexual orientation, or any other aspect of the story.
It makes no sense, and ruined the story for me. If the female lead and the male lead simply remained, as the title suggests, Best Friends, rather than shoehorning in a romance between them which seems so out of place it halts the whole story, then I would have loved to continue reading. As it sits, I don't know if I can. I will probably continue to do so, in the hopes that the entire storyline of them being anything more than friends is summarily dropped (I can hope), but I'm no longer optimistic.