This page of the guide to Animal Crossing New Horizons describes Wisp, a ghost who will need your help.
Wisp is a bit of a sad ghost. Not only does he not know how to scare anyone, he also gets scared easily - as soon as we meet him, he falls apart in fear. If we can help him find his pieces, Wisp will reward us with a little gift.
How to help Wisp?
To help Wisp regain his normal form, we need to find his five fragments. Just walk around the island and you will catch them pretty quickly (you need to use a butterfly net). As for a reward, Wisp can give you something expensive or something you don't have. It's better to choose the latter option, because the ghost doesn't know much about the value of items, so by choosing the "more expensive" option we can get not only a cheap gift, but also one that we already have.
When does it show up?
Wisp visits player's island on random days in the evenings. You can meet the ghost from 8 p.m. to 4 a.m.
Where to find it?
Wisp appears in random places, but the ghost stays away from the main square, it prefers more secluded places.