The 13th Footprint manga volume 2 features art and story by Kei Sanbe.

After yet another unsettling postcard arrives at the Togawa house, the search for answers begins, and it all leads back to Touya! He recalls having a similar postcard in his youth, but how long ago was that? That's when Touya remembers that he gave it to someone - a boy he knew named Sora. But that was ages ago! Could Sora, someone from the furthest reaches of Touya's past, really be the mysterious sender...?