Kai of Exo, and his Baby sister Kim Do Rim, Was very close. Kai was very protective over her, for she was his Baby Sister. 

So when Kai has heart cancer, Worried about His Sisters Reaction, Kai doesn't end up telling her.

It came to the point that the doctors couldn't treat it anymore. So on his last day alive, Kai writes a letter to his sister, and Hands it to Sehun, Telling Orders to give it to her when the time is right.

When hearing her brothers death Do Rim thinks back to the past.. And At first she was cursing him.... (She hasn't gotten the letter from Sehun Yet) But in the end.... She starts to forgive her big brother for leaving her behind..
 

But she feels alone now, pushing everyone away... And Kai could only watch from the side lines.

 

" Listen Kid. Your Not alone, Even if you can't see Me.

I'm still here. Just look in your heart.

Kid Look to the stars, The sky, the Clouds and tell me what you see."