Hellen Gravely is a slim woman with beehive-shaped hair, the face of a middle-aged woman, and a white and blue dress that hides her ghostly tail. Later on, it's revealed this is just a facade to hide her true form, which she generally only reveals when she gets really angry: a much older appearance, similar to an old woman, a sharper chin, defined cheek bones, a crooked nose, messy hair, red eyes, and her dress becomes torn slightly around the bottom. She hides her true appearance using a special makeup.
Hellen Gravely is a huge fan of King Boo. Sometime after the events of Luigi's Mansion: Dark Moon, she lures Professor E. Gadd to her hotel, then captures him and steals his ghost collection, including King Boo, who she releases.
Hellen Gravely sends an invitation to Luigi, Mario, Princess Peach, and the Toads to come to her hotel for a vacation. Upon arriving, she wears sunglasses to hide her ghostly features, and welcomes them warmly, showing them to their rooms on the fifth floor.
Little do they know, it turns out to be a hoax, as after dark, the hotel takes on a more creepy look. After Luigi investigates Mario and Princess Peach's rooms to find them gone, he encounters Hellen Gravely coming off the elevator. She reveals that she is in league with King Boo, who is intent on trapping Luigi and the others in paintings, having captured Mario, Peach, and the Toads. Fortunately, Luigi is able to evade capture by going down a laundry chute.
Afterwards, Hellen, and her ghostly pet cat, Polterkitty, monitors Luigi's progress from her office in the Main Observation Room on the hotel's fifteenth floor, the Master Suite, getting angrier as he frees his friends one by one and recovers the missing elevator buttons.
Eventually, Luigi reaches the fifteenth floor; shortly after getting to the Master Suite, Hellen appears on a projection screen, outraged that Luigi managed to get that far, and commends him for making it to the fifteenth floor, then challenges him to come and rescue Mario, so she can capture him herself and turn him over to King Boo.
Acquiring the four keys needed to access the Main Observation Room, Luigi enters to confront Hellen. She lashes out at him for trashing her hotel, capturing her staff and her Polterkitty, getting so angry that she unintentionally reveals her hideous true form. Calming down and deciding to fight Luigi herself, she closes the door to her office and turns the room into an arena.
During the battle, Hellen activates rotating lasers to attack Luigi, which he can avoid by using Burst. However, sometimes she will activate three laser beams that he can't jump over, so he can only avoid them by running. Afterwards, she activates four colored lasers that are controlled by switches underneath the floor, requiring Luigi to send Gooigi down there to trip the switches, deactivating the lasers. Hellen counters this by flooding certain sections with water; the sections where the water is going to flow are illuminated with blue lights, serving as a cue for Gooigi to get out of there. While the colored lasers are active, Hellen follows Luigi trying to hit him with her mirror, which she uses to protect herself from the flash of his Strobulb. Luigi must dodge the mirror, causing Hellen to get stuck, giving Luigi the chance to use the Poltergust G-00 on her. She has 600 GP, so it'll take a few rounds before he finally bags her. When he does, she asks that he give her a second to put her makeup back on, then admits defeat.