Pennington (or, as he prefers to be called, The Penguin with The Improbably Large Brain) is a Bumpty detective from Poshley Heights in Paper Mario: The Thousand-Year Door. Mario first meets Pennington on the Excess Express, where he takes Mario (whom he mistakenly refers to as Luigi) under his wing. Pennington and Mario must solve the many mysteries that occur on the train. Of course, Mario solves all the mysteries with no help from Pennington, who often takes the credit for solving them and treats Mario as an apprentice. His catchprase, "Rudimentary," bears resemblance to Sherlock Holmes's, "Elementary."
During the ride on the Excess Express, if Mario talks to Pennington before warning the conductor about the sticky yummy threat, he says that he is on his way back home. After the Stew Pot is stolen, Pennington gathers the train passangers to find the thief, and assumes that Mario was the suspect. Pennington then has Mario find the real thief to prove his innocence, which turns out to be Heff T. who stole it because he was hungry. Pennington congratulates Mario, and begins to work with him to find out who left the threat about a sticky yummy doom.
The next day, the culprit is revealed to be a Zip Toad imposter, which Pennington takes into custody. However, after getting off the train at Riverside Station, the imposter is revealed to be Doopliss who reverts back to his normal form and escapes from Pennington. During the last day on the Excess Express, Pennington is among those who is captured by the Smorgs. Mario destroys the creatures and saves Pennington and the others.
When the Excess Express arrives in Poshley Heights, Mario heads to Poshley Sanctum to find the Garnet Star. However, the sanctum curator is unavailable and the entrance is locked. Though this is to Mario's dismay, Pennington soon arrives and unlocks the door, revealing that his real profession is being the manager of the sanctum (being a detective is just a hobby of his).
Although he is the curator of the sanctum, he does not know where the Garnet Star is located. The only item on display is a replica of the Garnet Star (which Pennington calls a "red herring"), probably to protect the real star from falling into the wrong hands. This red herring is taken by the Shadow Sirens, although after Mario retrieves the real Garnet Star, Pennington allows him to take it and puts another fake on display.
When Bowser and Kammy Koopa go to the Poshley Sanctum to steal the Garnet Star, Pennington catches them and informs them that what they have is a red herring, and that he gave the real one to "Luigi". After Bowser and Kammy depart to ambush Mario in the Palace of Shadow, Pennington condemns Mario for his efforts.
When everybody is cheering Mario on when he's fighting the Shadow Queen, Goldbob, his family, and Toodles are shown cheering him on, and they call him both "Gonzales" (Mario's Glitz Pit pseudonym that some of them knew him exclusively by) and his actual name, which causes Pennington to be surprised and ask if it's Mario or Gonzales, and if it's not Luigi. Even after Mario beats the Shadow Queen and returns to Poshley Sanctum, he first calls Mario "Luigi" again but quickly corrects himself.
- (Excess Express) That's Pennington, a train passenger. He SAYS he's a detective, but I don't know... I mean, why do detective types ALWAYS show up when something goes wrong, huh?
- (Riverside Station) That's Pennington, a train passenger. He SAYS he's a detective, but I don't know... Not to be, y'know, totally catty, but what kind of detective LOSES a suspect?
- (Poshley Heights) That's Pennington. I guess sleuthing is just his hobby. He's the sanctum-keeper. I knew that detective bit was fishy... but I guess a little sleuthing on the side is OK...
- In Super Paper Mario, a plush doll of Pennington can be found in Francis' room.
- Whenever Pennington says the word "Luigi," it appears in green text, as previously seen in the Mario & Luigi series, where Luigi's and Baby Luigi's names and nicknames are almost always written in green.
- When all the game's characters are cheering on Mario before the fight with the Shadow Queen via the Crystal Stars, he says "Go Luigi...err...Mario?"
- Pennington's character may be a reference to Hercule Poirot from 33 of Agatha Christie's murder mystery novels.
- In the Italian translation, his moniker is "the penguin with the little pink brain cells," a reference to Hercule Poirot's "little grey brain cells".