She isn't just vanilla, she's a massive Mary Sue. The most beautiful, the kindest without a hint of darkness, looks like she has no fat on her body but still has big boobs, no muscles but better athleticism than all the boys, can knock out a boy twice her size with a single dodge ball throw, was awful in Maths but after a single lesson got 100/100, even the tutor Keisuke only got 95, was socially awkward but turned out to be a great actress immediately, hasn't had friends in years but could fix Irie's problems by herself and then improvise the play to success and then everyone is her friend.
Is there a single problem with her or is she just God's perfect little Angel, all the guys want her, the girls are just jealous of her. Even her childhood friend group breaking had none of her fault, it was everyone else that was in the wrong. Good ol' Kusunoki has never done anything wrong.
So when Hebikawa says that she wants to bring out her darkness, we cheer because we also want Kusunoki to have any fault in her character at all.
Not to mention, her character development was linked to her making friends other than Keisuke by herself, that's done now. So what is her character left? Hebikawa can be the focus of dozens of more chapters because her character is more complex, and she can actually get character development, not to mention that her interactions with Keisuke are always way more interesting than anyone else's. Keisuke himself is only an interesting MC when he's with her, he's just a dense guy in front of Moe and just a sad sack in front of Kusunoki.