Wisp

Wisp is a ghost who has only appeared in Animal Crossing: Population Growing and Animal Crossing: City Folk. He appears in the late night hours and will grant you a wish of three to choose from after helping him: Squish all the cockroaches in your house, give you a rare piece of furniture, or pull all the weeds in town.

Animal Crossing: Population Growing
In Population Growing, Wisp is found in the late night hours. When you are in an acre where Wisp is at, he will call to you in a faint voice. Running around that acre will eventually make Wisp appear. Wisp will then explain that he must find the 5 spirits and catch them before 2:00 A.M. or else he will be punished. Once the Player finds all the spirits and talks to Wisp, who is hiding in the same acre he was found in, Wisp will grant one of the three wishes and gladly dissapear.

Animal Crossing: City Folk
Wisp returns in City Folk after being absent in Animal Crossing: Wild World, but has a genie-like appearance, possibly because instead of finding spirits, this time the Player has to find the Empty Lamp. Wisp is found the same way as in Population Growing. He explains that he must find the empty lamp before 2:00 A.M. The lamp will be lying on the ground in a random area in town. Once the Player finds the lamp and talks to Wisp, the ghost will gladly go inside his lamp and requests to take you to your attic. If the empty lamp is rubbed in the attic, Wisp will pop out and grant you one of the three wishes and dissapear.

Trivia

 * Wisp resembles a Boo, an enemy from the Mario series. Wisp has fat lips and lacks the distinct Boo-like eyebrows, but he has the spherical body and pointed hands of a Boo.