Look, I know I joke about Hina coming in on the rebound all the time and gush over her as my favorite character, but I'd like for you to allow me a few moments of Real Talk™ here.
Hina and Taiki are in high school. They are kids on the cusp of young adulthood, but they are very much still kids. Humans figure out a lot of shit at that age, and for all that knowledge gained is helpful, it is not always a pleasant experience learning it, nor is it always clean. Hina fell in love with Taiki, shot her shot, and was rejected. She thought that she could handle that and move on, but doing so is not easy. Lying to yourself about where your feelings stand just puts yourself in a world of hurt. I think Hina has been suffering with this. She knows good and damn well that making another move on Taiki right now is not the "right thing" to do, but keeping it bottled up is eating at her and she doesn't know what to do. Do I condone making moves on someone in a relationship with a friend? No, of course not. Do I understand why she would do so? Absolutely. Cut the girl some slack; this is an important, painful lesson in the making.
...still best girl though obv