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Stewart is a Cheep Cheep in Paper Mario: The Thousand-Year Door. He is the Blimp Conductor of Rogueport to Glitzville; his job entails checking the passenger's tickets and allowing them onto the Cheep-Cheep Blimp.
He loved the sea as a child and wanted to be a sailor. Goombella says his dream kinda came true because he is now sailing the skies when tattling on the one in Rogueport. In Glitzville, she breaks through the fourth wall, then notices that she did.
Strangely, Stewart is seen remaining on land when Mario departs on the Blimp, and is then seen at the destination upon arrival. How he reaches the destination before Mario and the blimp is never revealed, and may just be an oversight on Nintendo's part. There might be one Stewart at each of the blimp's stops.