During their class trip, Eun Ji Ho drunkenly admits to Yoo Chun Young that he's held feelings for Dan-I for over a year and a half. He confessed to her but she didn't understand he was being truthful and thought he was joking.
Here's another excerpt of him dealing with his unrequited crush:
“If having a crush on her took place suddenly, my feelings should cool down quickly or so, but it doesn’t work like that. I was so annoyed at first but the only thing that comforted me was this. No matter how many times you guys act like that to Donnie, she will never see you all as a guy. This… only this comforted me. I hate myself being such a loser relying on that to console myself, but you guys acting like that is even worse, so I guess I’ve gone through my stormy period of adolescence quite enough.”