Hmm, a lot of people have been asking for enemies/rivals to lovers yuri lately and here it comes. Implied anyway.
"Two equal sides of a trianlge" as in equally disconnected from their target. I very much enjoyed the debate over why Koyuki is closer to them, as it compared a lot of general romance tropes and also showed their flaws. Ame knew a lot about Koyuki, but there are also many aspects they dont show each other exactly because they are so aware of each other's comfort zones. I especially love that Ame's reaction implies she was aware of the sport club senpai in middle-school but never connected the dots that Koyuki liked her. She was too confident about her position as childhood friend. The new girl was just projecting her attraction on Koyuki's actions and assumed her kindess meant romantic interest. Though she had more in common with Koyuki she never considered that this just made it easier to become friends.
Koyuki's kind of a bad friend, ngl. Being uninterested in her friends' squabbles and sticking to her phone bc she's focused on senpai is one thing but just ditching them without even asking if it's okay and no explanation at all is just kind of a dick move
They see each other every day, so she most likely assumed if they want an explanation they can just ask her directly. She went out of her way to cancel the plans as soon as she made a promise with her senpai, which is actually pretty thoughtful. She could have just told them the next day, but the more time they have to change plans the better. Koyuki couldnt have known that the new girl secretly booked a hotel due to overwhelming thirst.