Here are three possible ways I can think of:
1. The person who had the encounter created a device that is able to detect ghosts or make them visible. Before ghosts could not be detected and were only a myth. This would enable the story to take many directions, as the ghosts do not necessarily have to adhere to common myths.
2. Ghosts existed all this time, but they generally keep to themselves. This would need an expaination to justify why this ghost is different and wants to meet the protagonist.
3. The only people who can see ghosts are old people, and there are not many ghosts. When they tell others, that they saw a ghost, the story is generally discarded as fantasy. This would make the story kind of sad as the protagonist is not believed and could feel bad about the fact that people don't trust them.
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