Why are some acting like this isn't common in real life? Realizing late that the friendship you had with the opposite sex turned into love out of nowhere is common, especially to teenagers, and usually teenagers easily get jealous if their partners are close friends with the opposite sex; that's why many people with opposite gender gets sour in their teenage years because of unnecessary jealousy of their partner. So, yeah, I get what Kusunoki feels.
And for the record she's not that late. Only one girl is actually aiming for the dude with pure intention, and Hebi only wanted to smash that "mask" that Kusunoki wear even if it ended up traumatizing Keisuke again.