Same here~
But, to continue, Kokorogi is definitely on Yuuichi's side (I feel). Like you said, she gives off soft yandere vibes, so I'm sure the extent she's willing to go for Yuuichi to make him happy is tremendous to the point of absurdity. There's no way she'd betray him.
She may be over eager to please him, and stupidly in love with him, but she isn't *actually* stupid.
It's been shown before that both her and Yuuichi are amazing actors when they want to be. Kokorogi has an especially easy time fooling others when they assume that they know her personality and then oversimplify her (like Saika is doing) to be a naive, timid, and dumb chick.
I'm going on a rant.
To get to the bottom of it, she's playing Saika and has been on Yuuichi's side the entire time (though her telling Saika that she wants to help and grow closer to Yuuichi was not a lie).
I feel like Yuutori is so obsessively in love with Yuuichi that the only man she could ever "worship" is him.
She might have gotten close to Kamishiro Saika of her own accord, because she truly believed that he would help her get Yuuichi's romantic attention, but -- at the end of the day, I feel like she wouldn't do anything that would make Yuuichi ultimately unhappy. She knows that he cares for them, and wants to "complete" the game, so I don't think she'd help sabotage him by passing valuable information to Saika without Yuuichi's express permission.
In other words, this is a semi-Yuuichi plan with a semi-independent!Kokorogi twist.
... I still don't think I like her very much though -.-;