If you remember at the end of chapter 19, Hajun got a text from someone and he seemed upset at it. There's no doubt that the text was from Hayeol. Perhaps Hayeol approaching Danwoo was his way to try and reach out to Hajun? We'll have to see the next chapters to truly know his motives...
Based on this most recent chapter, it just seems to be possessiveness. I really hope Danwoo doesn't end up with the twin who inserted himself into his brother's romance.
If you remember at the end of chapter 19, Hajun got a text from someone and he seemed upset at it. There's no doubt that the text was from Hayeol. Perhaps Hayeol approaching Danwoo was his way to try and reach out to Hajun? We'll have to see the next chapters to truly know his motives...