Despite what was written about the triangle relationship between Ilbaek, Sambaek and Soo Jeong earlier on, it was obvious from latter episodes until episode 99 (most evidently) that it was really just Sambaek and Soo Jeong.
Despite what Ilbaek does, Soo Jeong just sees him as an elder brother and nothing more. Some thing are just not meant to be no matter how hard you try. In episode 84, when Ilbaek learnt about Soo Jeong’s wish to hear someone sing Aubrey to her, he tries to find the opportunity to sing to her. But right at the death, totally by coincidence, it’s Sambaek who sings it to her instead.
Sambaek learns about Ilbaek interest in Soo Jeong and thinks of ideas to help him get her affection. But it always backfires and almost landed Ilbaek into her foster brother. Sambaek decided to do the extreme by not hanging out with them in episode 98 This is when he realises something is missing and eventually reunites with them. His feelings for Soo Jeong gradually materialising.
Then in episode 99, when Soo Jeong tells Sambaek about the possiblity of his father looking for them, he was confiding in her only and no one else, not even Ilbaek. Right up to getting a present for his “father” and meeting him, Soo Jeong was always by his side. At the end, when Sambaek tells Soo Jeong that that wasn’t his father, nothing could describe that scene but pure agony for Sambaek. But that brings them even closer together.
Soo Jeong was on Sambaek’s scooter earlier as she sat behind, just holding on to his waist using her fingers. But at the end of the episode, with both learning more about each other, Soo Jeong has her hands around Sambaek’s waist snugly, looking mightily blissful. Sambaek didn’t reject, taking it naturally. The question now is telling Ilbaek about it when they get together eventually.
In episode 100, you could see Sambaek trying to fight back his growing feelings for Soo Jeong. He tried to bring Soo Jeong to a restaurant where Ilbaek was waiting. Enroute, he snaps pictures of Soo Jeong upon her request and it would be a valuable collection to him later. While taking the lift with Ilbaek waiting in the restaurant, Sambaek and Soo Jeong gets stuck in the lift and their feelings grow as they spent quality time together. Later Sambaek managed to find the confession letter Ilbaek lost which was for Soo Jeong, he tries to be the good guy and sends it by post. But it just gets lost along the way.
It looks like Ilbaek is doomed to live a pitiful life unless they bring in someone.