Kaori eventually dies due to the surgery, but leaves a letter to Kosei, which is given to him by her parents at her funeral. The letter reveals that she was in love with Kosei the whole time, and that her supposed feelings towards Watari were fabricated in order to get closer to Kosei without emotionally injuring Tsubaki, who also has limerent feelings towards Kosei. Although at first consumed with grief, Kosei finds comfort in Tsubaki, who promises not to abandon him.

What's the best answer to this question: does kaori die in your lie in april manga?
yes