Trappers wait in front of doors or against the wall in narrow hallways while invisible. If Luigi comes near them, they will reveal themselves and grab him with one of their tongues. To prevent Luigi from taking damage, the player must call out Gooigi and flash it with the Strobulb. Both Luigi and Gooigi must then pull on its tongues together and slam it to defeat it.
Luigi can also reveal them with the Dark-Light Device, allowing him to defeat them without having them grab him. If Gooigi bursts next to a Trapper that is attacking Luigi, it will drop Luigi and return to its invisible state. On the 15th Floor of the Last Resort, there is a Trapper that, if not sucked up after being flashed by the Strobulb, will become invisible and move to a different spot on the floor.