
Could be both, people aren't black and white.
On one hand he's very indifferent to romance and therefore did not mind marrying her for his family and his dad's savior happiness, he enjoyed making the people important to him happy through something that he himself didn't feel a need for. But at the same time he enjoyed how Yumin made him feel, he was loved, cherished, taken care of emotionally. He would go on walks with someone who cared about him and who would match his pace. Which may be something he had never experienced being very popular with a lot of people viewing him as "the perfect guy" or woman wanting his full attention.
Overall I dislike him but I wouldn't call him a red flag (at least not right now), a yellow flag tho

I get what you're saying, and it's true we don't fully know his intentions yet, but if he is actively trying to get back with Yumin -- knowing that he has never reciprocated her romantic feelings -- then I won't be able to give him the benefit of the doubt anymore. He may have felt like he had no choice the first time but he doesn't have that excuse now
I don't understand. I thought Yiseon was relieved they'd broken up because he felt pressured to date Yumin. Even the getting back together stuff felt kinda forced. Shouldn't he be happy she's moving on?