Yuu passed away in a car accident when he was very young.
The circumstances of his death aren't very clear, but Chihaya was present at the time and was unable to save him. Due to this, she went through a period where she blamed herself for his death.
Growing up he loved to listen to Chihaya sing, and would often ask her to sing for him. She is also witnessed singing in order to cheer Yuu up after he broke his toy. This is the reason Chihaya believes that she has to sing for Yuu.