Bro remember in Chapter 3?

Sanyu knew about it and I guess he told Uion that's why they have that conversation before Shiyeon drove the van and had an accident. I don't know actually... I'm just really summing up the pieces that's why Sanyu is protective of him especially around the blonde guy on the earlier chapters. I guess he was guilty for not doing anything so he tries to protect him in a way he can.