I reread the manhwa because i was desparately curious about who is the girl in Mincheol's dream and tthis is why i think it's Haneul.

1. The girl in his dream mentioned something about pretending to care as a friend and as far as we know Mincheol doesnt really has a lot of close friends that are girls except Haneul. This mean Mincheol might have a feelings for Haneul for ages.
2. The girl in his dream had a long straight hair and we all know that Haneul also used to have longer hair before. Though at first i thought the girl in his dream was the girl in last page of chapter 27, but she has slightly wavy hair and lighter coloured hair.
3. The promised thing she mentioned in his dream could mean that Mincheol did a promise to himself not to ruin their friendship but he did eventually by having s*x with her.

But there are might be more complicated things that happen to Mincheol and Haneul and Haneul might not aware of so Mincheol is keeping it a secret so they can maintain their friendship. I could be wrong tho.