Yanagi-kun no Himitsu

Waka Ichimiya is a high school girl without much of a social life, and she's worried that she is slowly becoming a total outcast. She secretly longs to be more like Shuuichi Yanagi, the cool boy who is always surrounded by a crowd, but she can only look at him from afar... But one day, while leaving school, she helps out Yanagi-kun's younger brother Haruka without knowing who he was, and it's the start of an unexpected relationship!

(Source: Shueisha, translated)