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Ghosts' hearts serve as the HP of the ghosts in Luigi's Mansion. When the player flashes Luigi's flashlight on a ghost, it gets stunned and shows its heart for some time. After doing so, the player can suck the ghost up in the Poltergust 3000 while its heart is revealed. If an element protects a ghost's heart, Luigi must first use the element's weakness to expose the heart.
Portrait ghosts are harder to catch as they do not show their hearts as easily. The player needs to find a way to make their hearts vulnerable so that they can be sucked into the Poltergust 3000.