Who finds the letter at the end of Gakkougurashi? At the end of Gakkou Gurashi, a person finds one of the letters that were carried by the balloons. 

Who was that? Is it someone random or is it someone we know?
She appear only in manga. She's Touko Deguchi. She's an university student.

 

From Wiki:


  Touko made a cameo appearance in the anime, where she picked up Yuki's letter


She appear in manga (Chapter 37). After the School Life Club left the school, they decide to go to university and Touko is one of the main character in this arc. She's the leader of their group. 

In manga the letter was found by someone else, not her.