I wouldn’t say he’s outright evil as many other comments say, but he’s definitely under a lot of pressure and is making the wrong decision instead of coming clean.
The professors praise his photos and his talent, and giving him opportunities. So it’s difficult for him to give it up and let someone else take that spot. Using her was an accident, and he’s trying to avoid that photo getting attention. But the professor liked it too much. From here on it will be interesting if he starts avoiding Yeseul, or make the bad decision to start using her.