Sakura, Saku.

A mysterious boy comes to Saku Fujigaya's rescue when she falls in on a train, but he leaves before she can thank him. After this experience, Saku never ignores strangers in need of help to emulate the boy who helped her.
Saku would like to thank the boy who helped her, but all she has is a note signed "Ryosuke Sakura." She discovers that a boy in her high school, Haruki Sakura, has an older brother named Ryosuke. She asks Haruki to deliver her thank-you letter to Ryosuke, but why does he refuse?
(Source: Shojo Beat)
Note: Includes the one-shot "Otome no Itari" in volume 5, plus prologue and epilogue chapters.