that's our girl. bet this is intended to pressure Yae into giving up the one giving him instructions, either deliberately (some sob story to try and stay in Yotsuha's good graces) or unintentionally (panicking, getting caught red-handed, etc).
alright folks, any ideas on who the mystery instructor is? 's gotta be someone from that same childhood who benefits from the mc being bullied to fuck and Yotsuha getting snatched up by Yae, so I'm gonna guess it's the girl behind Yotsuha in the picture.