beta
I'm team sunbae. My take on this story -
I think the other guy will serve as a catalyst for the relationship between the uke and sunbae. I think it's their love story. The author is purposeful in making the black hair lead seem like a villain because he is the final* antagonist. I think the uke will choose the black hair lead for a while, even when he finds out he is not human. He'll try to give him the benefit of the doubt and keep his promise with him.
Meanwhile the sunbae will slowly come to terms with his feelings for the uke and try to protect him throughout the story. The uke will go back and forth, and I believe the black hair lead will eventually try to harm the sunbae and the uke will intervene somehow. Things blow up further and the black hair lead will come to some self-actualization and realization that the uke isn't lonely anymore and has had someone, then he'll go back to where he came from.