By the end, Rin-Senpai proved herself indisputably by both words AND actions to be BEST GIRL for Taizo-kun (lucky bastard), with added upgrade to "Future Waifu".
All Together Now: "Isn't That Cute? ... BUT IT'S RIGHT!"
Hmm...So he went and got a girlfriend 'A month ago' despite the perfect girl being right next to him this entire time?
Well hey, at least it allowed for this sweet Vanilla loving to happen.
Great ending though. Well hey man, I'm sure that answer was obvious even without asking, but now that you've received an answer...You know what to do.