Grimmly only appears after lightning strikes Luigi's Mansion, causing a power outage throughout the entire mansion. Grimmly locks the door of the previously accessible Breaker Room, preventing Luigi from reactivating the breakers and restoring power to the mansion. Shortly after the lightning strike, Luigi receives a phone call from Professor E. Gadd, who informs him that Uncle Grimmly holds the key and that he prefers to reside in rooms with mirrors (in the 3DS version, Professor E. Gadd says "rooms with clothes and a mirror" instead).
Grimmly resides in the Wardrobe Room on the second floor, where he hovers in front of a mirror. Upon seeing Luigi, he attempts to scare him by lifting up his hand and wailing. During this movement, the ghost is vulnerable to Luigi's flashlight, allowing him to be sucked up by Poltergust 3000. After weakly struggling, Uncle Grimmly is eventually captured and drops the key to the Breaker Room.
Uncle Grimmly has pale blue skin, an oval head, small yellow eyes, and jowls. He has a brown coat with two opal buttons, a dark purple necktie and a boutonniere on the left side.