
He is not really red flag he was just too in love with Se-jin even in the first timeline and was hurt. He even told Se-jin in that time line that any excuse he will accept. He just wants to hear the reason why Se-jin stole the program and betrayed him but Se-jin out of shame he didn’t give Ido any reasons or excuses at all. So Ido thought the to Se-jin he is not worth making excuses for. Ido that time thought that Se-jin was also in love with him but because of what happened in his mind he realized that Se-jin only loves his family and the company. He even told Se-jin that “In Se-jin’s heart there’s only his father and Hea-sin. Where am I even there?” “I was never there.” That’s what Ido said and that’s when he turned from green flag to red flag.

There will be admission this time it is just that Ido refuse to let go the guilt of the past. That’s why the ending seems like an open ending. Both are healing together and even Se-jin said to Ido that “At the end of that memory we are happy.” Se-jin wanted Ido to stop torturing himself because of his mistakes during the first timeline.
Are we getting the story from his past life? I'm dying to know what happened during that time