From the Super Mario Wiki
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Pook is a young boy and a resident of Flipside, from Super Paper Mario. He is fascinated by sea creatures, and wants to be a sailor when he grows up. He hopes to leave Flipside when he grows up and listen to the quiet sounds of the waves at sea, every day. He used to have a pet fish he named Captain Gills that he found somewhere, and kept him in a cup. Captain Gills got too big for the cup so he searched for something bigger to put him in. Merlon generously gave him his Goldfish Bowl but not too long after Mario and co. arrived in need of it. Pook agreed to give them the bowl, only if they would give a new home to Captain Gills. The player then must go stand near the edge of the water to release Pook's goldfish. If Mario, Peach or Bowser talks to Pook at any time before entering Outer Space, he will talk about a problem that keeps getting bigger and bigger. This "problem" Pook is speaking of may be Captain Gills, as the fish grows in size every day.
After Chapter 4, Pook states his wish to become a sailor one day and meet Captain Gills at sea, completely ignorant of the fact that the goldish was released into Flipside's underground canal and, according to Tippi, no longer needs little kids in his life.