Ikotsu no Tabiji

The night of his brother's funeral, Kasuka Tokio interrupts a young man in the act of stealing his brother Hajime's ashes. The man, Kuroki Ren, claims to have promised to go on a trip with Hajime, and asked for his bones so that he could take them on the trip instead. Tokio, intrigued by him, agrees to lend a bone to him, under one condition...

(Source: Printemps Publishing, translated)