Miss Petunia, the Bathing Beauty is one of the portrait ghosts in Area Three in Luigi's Mansion. She can be found on the second floor of the mansion inside the bathroom shower. Luigi must suck up an Ice Elemental Ghost and shoot it at Petunia in order to make her vulnerable and expose her heart. Then, he can suck her up with the Poltergust 3000.
A book can be read in Nana's Room about Miss Petunia saying how Nana pulled a prank on her; which hints that Luigi must use the Ice Elemental Ghost to defeat Petunia.