Kimi no Iru Machi: Tasogare Kousaten

Kimi no Iru Machi: Tasogare Kousaten is a prequel to the TV series. This was covered in manga chapters 17 and 18.

After being like worlds apart, Haruto visits Tokyo on a school trip and plans to meet with Yuzuki whom he dated while she was staying at his place last winter. But they miss each other, and Haruto ends up playing Nanami's boyfriend so she can impress her friend. But when Yuzuki sees Haruto and Nanami holding hands at the train station, she misunderstands it and starts thinking that she is the only one trying to invest in this long-distance relationship.