This scene right here. I think this is the first time in the series where Hiei is actually scared shitless. Yusuke can’t win against Sensui as he is and everyone but Kuwabara knows that. And both Kurama and Hiei know they don’t stand a chance either. The fact Yusuke is about to die is very real to them. It isn’t often Hiei shows any kind of emotion other than anger, but he is truly afraid here, and that still doesn’t stop any of them from going after Sensui to avenge Yusuke.
This is true loyalty, even though in this moment they have no faith that Yusuke will win.
This has absolutely nothing to do with anything, but that small clip of that monkey voice speech is creepy, and fascinating, and I just wanted to save it for reference, because dang, I want a character with that voice.