For all the dummies who needs this to be spelled out: Natsumi has been painting the nail of her left ring finger to symbolize a purity ring, meaning that even if she dates other people she wants to keep her heart set on Futaba.
Throughout the manga it's been portrayed as a fool's errand and she herself realizes that it's immature and naively idealistic, with the way she talks about her feelings like a stubborn childhood crush and uses self-deprecating language to undermine it because she already knows Futaba will never love her back, and she knows she's being childish by clinging on to the hope that it can change somehow.
Her tendency to call Yuuto, a teenager, "much more of an adult" than she is, is an acknowledgement that she knows the feelings she's had for Futaba (which started in middle school) is overstaying its welcome and she's getting tired of pretending it's anything worth pursuing, which causes her loneliness and grief (the smoking and moping and crying). Not to mention the fact that she's constantly drowning in her own thoughts despite never even telling Futaba about any of it.
As for the people who were duped into thinking the unrequited yuri rival subplot will ever go anywhere in a het romance story, try and recognize the patterns. It's always introduced to add drama with no real consequences because despite the ensuing conflict, the straight girl will never be swayed. It's an annoying trope that shows up too much in romance plots (usually shoujo) and all it does is pad chapters so the authors can milk their story a bit longer. I've literally ranted about this.
Kou will never date Usagi, Sawada will never date Hori, Char will never date Juliet, Kanbaru will never date Senjougahara, Gundou will never date Tomo, etc.
That being said, this author is kind of a hack, so they might try to pull something out of their ass to turn the ending into yuri fap fuel for their fanbox. Or at least a fanservice chapter for the tankoubon.
Natsumi has not been "Stolen away" from Futaba by Yuuto. She's literally, right in this chapter, saying that she is her friend, which is more of a resolution for Natsumi and literally a non change for Futaba who doesn't know that Natsumi ever had a thing for her.
Crazy how falling in love doesn't mean you have to stop interacting with anyone else entirely. People can be friends.
Manga protagonists are so dense that their closest friends can overcome a whole-ass story arc involving their friendship over the course of a decade and they'd still have no idea anything is happening.
