In the end, when all this mess is over, all I can see is Youhei getting the short end of the stick.
Kei is messing with both of them, but seems to like Youhei a bit.
Yui, on the other hand, realised that she has no chance with Kei, and now might try to make Youhei fall (even more) for her, in order to steal him from Kei, as some form of "revenge".
Youhei will think Yui has actully moved on and is into him now, only for, in the end, get crushed by the truth that it was all just to make Kei jealous.
And the craziest thing? All of this might be beneficial to Kei, because he will pick up a heartbroken Youhei and give him the love Yui failed to give.
This... is not gonna end well.