Responses
FRRR like it’s jus makes more sense that it’s sihyun
It’s already confirmed that Yunjae isn’t the friend. He looks completely different in that picture than Sihyun as a child. Hajun just doesn’t remember because he was traumatized by getting injured so badly. This was literally explained in an episode. He sees Yunjae and mistakes him for the friend he had, but Yunjae is moving so he forces him to take that picture with him and then they happen to reunite in high school.
I don't know why but I really don't believe that Yunjae is Hajun's childhood friend. My gut feelings tells me that is it Sihyun. And that Vulture head said something about someone being a "trigger" really really bothers me and I really think it pertains to Sihyun. And I think that Sihyun's codename "Red Candy" is pertains from that scene at their childhood.