I should've killed you when I first met you

Led loses his will to live when he realizes that everything in his life is messed up after being diagnosed by a rare disease. When he tries suicide, he is saved by a mysterious girl who does not tell anyone about her identity. She changes his life and makes him feel lots of different feelings which he did not think that he would be able to feel in his lifetime. But who exactly is she? And why did she save his life? Everything is revealed as the story unfolds itself