I mean, I feel for her cause, you know, who wouldn't fall for Kayo? And rejection sucks, but... She had the entirety of high school AND university to confess properly and didn't? Girl, you snooze, you lose. And finally trying that shit right when your friend is incredibly happy in a healthy relationship is big oof territory no matter how genuine your affection for Kayo may be.
But I'm sure that, unless she suddenly disappears from the story completely, the author will find her another person eventually. It's always like that when they take the time to push for more reader sympathy with things like this flashback.
Also, people saying they don't hate her 'anymore' are wild. Not cause they should hate her, but because there was no reason to hate her in the first place. You can dislike the one thing we saw her doing (like I clearly did, I mean, look at my profile pic and imagine my reaction lol) without immediately hating the whole character. More often than not, people declaring their affection for others are just looking for happiness themselves, not exactly aiming to destroy other people's romantic lives.