Yuu is Chihaya's late younger brother, who passed away in a traffic accident some time before the start of the series The iDOLM@STER. His death weighs heavily on Chihaya, and is implied to be a factor that strained relations in her family.

Chihaya fondly remembers his enjoying her singing, and telling her to sing more as he doodled in a sketchbook. She keeps a photo of herself with her brother, taken when they were young.

(Source: first-hand from watching The iDOLM@STER)