He is a red flag because while he claims to want a FWB relationship, he is acting the opposite way, making her feel cared for. Notice that every time Nabi tries to break things off or starts to pull away, he starts acting really nice.
FWB do not give each other gifts, they do not cuddle, they do not go one dates.
If he wants to be FWB, he can make that clear but he is purposefully blurring the line for Nabi so that she keeps coming back. Then when she wants more, he immediately draws the line and it becomes her fault. That is not how someone who wants an open relationship behaves. He's manipulating her because he knows he can.
Many people seem to think this is because of his attractiveness but it really isn't. It is his charm. Attractiveness alone is not enough to string so many girls along.
@Anofferhecantrefuse said he isn't your typical jerk who hides behind a constructed personality to attract and deceive people for achievements and gains but I have to say, really?
Because from what I can see, Park Jae Eon seems to enjoy the thrill of the chase. He enjoys seeing these women bend over with a little twitch of his fingers. After Nabi and that junior end things, PJE tries to win her back and his behavior is eerily similar to that junior for a while. He saw that Nabi was attracted to the junior and acted in similar manner so that she crawls back into his net. In fact, he does this right after the junior rejects Nabi and she's feeling a little vulnerable, how is that not deception?
Well this has been longer than I thought it would be but these are my thoughts.