I think he (Yuri) brain washed him (Yuya). I am a Danganronpa fan and I remember in Danganronpa 3: despair Arc, Junko brain washed class 1-A by making a brain wash video about there friend (Chiaki) being dead. Or back in yandere Sim, you can kidnapp the student and make them your mind slave. So, there are two ways how Yuri brain washed Yuya: 1: brain wash video. 2: mind slave.