Writing this since some people seem to be having a hard time keeping up.
After chapter 10 where Yeonduk finds out that Skyler has been using him, he gets set on seeking revenge. He stops helping Skyler with his mother, doesn't relay the messages his mother sends properly to get him in trouble on purpose (when his grandmother came and he wasn't there, he had his card taken from him because of that), then used Skyler's ex to get to him, giving her chocolate saying it was Skyler who sent it, making Skyler looks like he cheated on his current girlfriend.
Then ever since the Cloudy Sky dude talked about distorted memories, Yeonduk has been restless, questioning his old memories with Skyler. And now he's going for a checkmate, trying to reveal that Skyler smokes to his mother.
