Kruller

Kruller is the third boss encountered in Luigi's Mansion 3. He is encountered and fought on Floor 3. Defeating Kruller rewards Luigi with the elevator button to the second floor. His name is a pun on "," a type of donut and a reference to the stereotype of police enjoying donuts.

Appearance
Kruller is an overweight purple ghost in a security guard uniform with sunglasses, blue eyes, and short black hair hidden under a security hat.

Luigi briefly witnesses Kruller as the latter is doing patrols on the third floor. After hearing noises, Kruller panics and tries to investigate the commotion, only to discover a mouse inside a flash vending machine. Dismissing it, he then enters his office, although he accidentally drops a key (the first key to a puzzle that results in getting the key to his office) when entering it.

Luigi can look into the office and can see Kruller reading a book while playing around with the elevator button.

When Luigi enters the office afterward, he briefly faints from shock at Luigi entering, before entering the next room to find a suitable weapon to defend himself (humorously getting stuck on his back mid-roll), ultimately getting a water pistol.

Battle
Because Kruller is behind a barricade, the player has to fight him as Gooigi. Gooigi has to flash at Kruller and then suck in his glasses before flashing at him again to stun him. Kruller will also shoot squirts of water at Gooigi; if any of them hit him, he will dissolve, wasting time. On the final round, he'll make it more difficult to suck in his glasses. He can also charge his water pistol to shoot a long stream of water across the room. Upon being defeated, Kruller will struggle to avoid being sucked in to no avail. Gooigi then retrieves the Floor 2 button, with it being humorously revealed that Luigi slept through the entire battle (as Luigi is unconscious while Gooigi is conscious, and vice versa). After defeating Kruller in two-player mode, Luigi, who was watching the battle from outside, takes all the credit saying that he did it, after which Gooigi humorously copies Luigi as he had actually defeated Kruller.

Trivia

 * Kruller is one of the few bosses in a Mario game, besides tutorial bosses in the Paper Mario and Mario & Luigi series, where the player is unable to lose; they must fight him as Gooigi, and if he is hit by his water pistol, Luigi can simply spawn him again.
 * Kruller is the only boss ghost in Luigi's Mansion 3 that is scared of Luigi.