To You, The Immortal不滅のあなたへ (Japanese); 致不灭的你 (Chinese); Fumetsu no Anata e; To Your Eternity (English)

An immortal being was sent to the surface of the earth and met a boy living alone in the middle of tundra. The being can take the shape of dying things, but only if the "impetus" is stronger than the previous one. What sort of experiences and meetings will the being encounter while living forever?