In the novel MC tried a few times to get a reason from him, but the only thing he stated is that he liked someone from that group. Even when MC and FMC announced to them that they are dating he still didn't want to tell him who he liked. MC is convinced that he likes FMC, but he never gave a hint, not even a sad look. He was happy for them...
It's not complete so no idea how it will end for him or Riho. They did make up, and are still hanging out as friends.