Yukina is pure and a complete contrast to Hiei, who feeds off of a bloody battle and the suffering he causes his opponents. He probably didn't want her feelings on what he was on his heart. Proving Yusuke and Hiei makes the fight even harder, but maybe he's amped up enough to where he makes Toguro look like a joke. Once Yoko knows it's him, I think Kurama will go to Koenma and ask to take Botan off the case, possibly behind her back (we need a smidge of angst!) which makes her feel betrayed, and like Kurama doesn't believe in her.

I'm not staying awake just for you ;3 I'm writing a post for someone else. You and that other person have some things in common, and one of them is replying quickly lol.