Flavio is a "famous", narcissistic, and naïve Rogueport entrepreneur who appears in Paper Mario: The Thousand-Year Door. He owns many ships, his favorite being the S.S. Flavion, which he and Mario need to use to travel to Keelhaul Key.
When Mario and his friends need to get to Keelhaul Key during Chapter 5, in order to obtain the fifth Crystal Star, they need to ask Flavio to ferry them there with his ship, the S.S. Flavion. Flavio reluctantly agrees to take Mario to Keelhaul Key under the condition that he can keep all the treasures of the pirate king Cortez. After finding an experienced helmsman to help navigate, Mario boards the ship and the group shoves off.
On the trip to Keelhaul Key, Cortez's forces attack the ship, causing it to sink. The group is eventually washed ashore on Keelhaul Key and they set up camp there, Flavio soon getting in an argument with Pa-Patch. As the two of them go off to fight, they encounter more Embers and hide behind Mario. Afterward, Flavio decides someone should investigate the island, choosing to stay behind and sends Mario off to explore in his stead. Once Mario finds Bobbery, he returns to Flavio to ask for a Chuckola Cola, saying it was Bobbery's last request. After giving Bobbery the Chuckola Cola and returning to Flavio, Mario and Bobbery tells Flavio about a mysterious skull rock near where Bobbery was found. Outvoted, Flavio partners with Mario for the moment until they reach the entrance to Pirate's Grotto, the puzzle to get into it being the Skull Gem Flavio owns and the puzzle in the song that he sings.
After Mario fights Cortez and obtains the Sapphire Star, he exits the grotto and finds out that Lord Crump and his forces are bombarding the island with their ship. Flavio suggests to ask Cortez to borrow his ship, the Black Skull. After some negotiating and Flavio giving up his Skull Gem, Cortez agrees to give Mario the ship. After a battle with the X-Nauts, Lord Crump decides to retreat, and Mario and co. go back to Rogueport.
Apparently, after the incident and bidding Mario farewell, Flavio got insurance for his boat wreck and became the second richest man in Rogueport.
According to the 7th issue of the RDM, Flavio was punched in the face by Zess T. after causing gourmet ingredients to spoil on the dock by holding up a crew with a two-day story. He temporarily lost his memory as a result. The issue also says he is twenty-five years old.
Flavio is constantly bragging about how brave and courageous he is, but he is actually a coward and tries not to engage in anything dangerous, instead trying to get Mario and his crew members to do it instead. He also gets mad at anyone who disagrees with him, mainly Pa-Patch, and he either snaps at them or challenges them to a fight.
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