Shinigami no Ballad.

Synopsis

Kantarou Ichihara is a boy in junior-high school, and the high school entrance examination period is coming up. His grandfather, who loved playing games, recently died, and while cleaning out his room Kantarou finds a message from his grandfather about a game involving a treasure hunt. Kantarou skips out on extra lessons at school to go look for the treasure in his grandfather's old country town, accompanied by his childhood friend Tomato. On the way Kantarou sees a young boy and girl several times, and in the end they lead him to where the treasure is buried, which turns out to be a large marble and a short note left by Kantarou's grandfather. It turns out that Daniel and Momo changed their appearances to those of Kantarou's then-young grandfather and his childhood friend in order to lead Kantarou to the treasure.

(Source: Wikipedia)