My theory on KBS Big

Because every Kdrama viewer has one.
First of all, my idea started after I read this theory online. This theory actually mentions 49 days as well, which involved a similar situation of a soul borrowing a body that is related by blood.
This is the first time I have ever heard of a savior sibling, but I think it is a plausible theory.
The Hong Sisters are known for making every detail relevant in their dramas. In My Girl, the mystery involved a cousin that was an underlying threat between the family dynamic and the relationship between Yoo Rin and Gong Chan. In You're Beautiful the mystery was Go Mi Nam and Go Min Nye's mother and Tae Hyung's mother's connection to their father. My Girl Friend is A Gumiho had an even more important mystery of how to prevent Dae-woong's death from happening in order for Mi ho to become a human.
A lot of people seem to think that Yoon Jae will die. This is possible, but considering the emotional pattern of the drama, I don't think that will happen.
Another thing about Hong Sister's dramas, the clues are always right there in front of your face. There's a reason they keep showing the book and the drawing over and over again, it plays an important role in the plot. If Yoon Jae did save Kang Kyung Joon's life in the past, there's a reason why Kang Kyung Joon is in his body now. It is a debt repaid for a life being saved.
Now, there are two theories going on about Kang Kyung Joon's body being in a coma. Either, Yoon Jae is dead, or Yoon Jae's soul is dormant in Kang Kyung Joon's body. But there's also the fact that in 49 days, two souls could exist in one body, if one soul was dormant.
I think this is the case for Yoon Jae, but not completely. Because Kang Kyung Joon and Yoon Jae are connected, it is possible that Yoon Jae is aware of what is happening with his body, but he is asleep in Kang Kyung Joon's body.
I know that a lot of people are upset that Yoon Jae will come back, but if you look at this from a writer's perspective, he has to. We have to learn the connection between Yoon Jae and Kang Kyung Joon. Whether they are siblings or donor matches, it is important to reveal the true reason behind their connection. Now it is true that Yoon Jae has not been painted as a pretty character. And even when Yoon Jae was Yoon Jae, we never saw him speak, act or react enough to discover what type of person he is. I guarantee that this was on purpose. In order to make Kang Kyung Joon appear more lively and fun, Yoon Jae has to be quiet and passive. This could be because of his role in the connection between him and Kang Kyung Joon.
All the crushes and relationships are no accident either. The foreshadowing of Gil Da Ran and Kang Kyung Joon came early in the first episode when Gil Da Ran noticed Kang Kyung Joon, but it is revealed in episode 2 that she didn't notice Yoon Jae at all. The relationship between Yoon Jae and Gil Da Ran is also a mystery because we only get bits and pieces through flash backs and Gil Da Ran's explanations that are bias in her favor. From what we have seen, I'm not convinced that Yoon Jae liked Gil Da Ran, or loved her. He cared for her, but he seemed to ask her out of guilt or duty more than anything. This is especially true if he cheated on her like the Hong Sisters are leading us to believe.
My theory is similar to Panda's, but I'm not convinced that Yoon Jae actually wanted to marry Gil Da Ran. The only characters that have expressed serious interest in her are Kang Kyung Joon and Teacher Na. Even in Gil Da Ran's colorful flash backs, Yoon Jae seems to be obligated to her but not interested.
Kang Kyung Joon will probably be Gil Da Ran's love interest in the end, but whether he is in Yoon Jae's body or his own remains a mystery. I doubt that Yoon Jae will die, but there is no doubt he will return and explain all the mysteries surrounding his proposal to Gil Da Ran, connection to Kang Kyung Joon, and decision regarding Se Young.