Hoshi wo Ou Kodomo

Asuna is a shy and diffident girl who lives in the mountain town of Mizunofuchi. One day, she meets Shun, a boy who claims to be from a mythical land known as Agartha. However, just as a bond is beginning to form between them, Shun vanishes. As Asuna wishes she could see him again, she encounters Shin, another boy who looks just like Shun, and Morisaki, a teacher searching for Agartha. The three of them set out for the legendary country, each with their own goal in mind—and their own lessons to learn along the way.

(Source: Yen Press)