Honestly, it's probably copium, but with how fraked out Yosiki was at someone just standing in the school grounds compared to how unphased he was with a hole in Hikaru's chest and all that shit oozing out of him and that in this episode he actually reacts like a normal human being when seeing all that freaky shit, I'm going to say that it's just a testament to how much he loves Hikaru that explains why he's so randomly okay with all the freaky shit surrounding the latter.