A ghost from the future

When someone dies their spirit is attached to someone. Whether that person has something in common with them or was close to them when they were alive only the being who attached their spirit to that person knows why. The only living person able to see the attached spirit is the one said spirit is attached too, though other ghosts are able to see each other. However the attached ghost cannot be seen by normal means like a living person, they are seen in the mirror behind the person, or in their peripheral vision. Perhaps the livings dreams show the spirit. This applies to the ghost's voice as well, whether the dream spirit speaks or the living hears them through a radio or other technology, certain spirits can be a voice in the back of the attached's head.

As for Shinichiro Sano, who was killed by his younger brother's friend while they attempted to steal a motorbike from his shop as a gift to his brother, whom he was already planning to gift the bike to, he was attached to a young boy who had an uncanny resemblance to him. Not only in appearance at that, he even had a similar personality.