Poor Yo he is being led around by a pair of toxic individuals. Let us remember that back in their primary school days Mitani kissed Kei with no issues even without even dating. I may not mean much but I think this is a prove that she really is not really very much into him, she probably is still attracted and likes the older version of Kei and is trying to separate Yo from Kei on here probably vain hopes to either revert back Kei to his older self (if she think that he is only like that because he is trying to attract Yo) or maybe she is even jealous of Yo and is on an "If I can't have Kei then I sure as hell wont' let Yo have him" campaign.
@Killthescribe dunno, I think this is all probably within his calculations. He said that he is not particularly interested in monopolizing him and he said he was gonna help Yo on getting on a relationship with MItani. So he probably foresaw such a situation happening eventually since the best way to break someones believes is to let them have what they think they want and then turn it on it's head