From the Super Mario Wiki
The Champ's Belt is a special item featured in the game, Paper Mario: The Thousand-Year Door. Mario received this belt after he had defeated Rawk Hawk in the Glitz Pit, becoming the arena's new champion. The Champ's Belt has a fake Gold Star implanted in the middle of it. Mario, however, cannot wear the belt.
When Mario first arrives at the Glitz Pit and watches Rawk Hawk defeat The Koopinator, he and his partner see Rawk Hawk showing off the belt. When they arrive back at the lobby, they are astonished to see the Gold Star attached to a cheap belt. Mario vows to make it up to the champion and fight for the star (or to steal it, but it all comes out to the same conclusion). Mario then finds out that it is not the Gold Star on the belt at all after meeting Rawk Hawk in person, and that he still has a big mystery to solve before he finds the real one. Unfortunately, when Mario and Mini-Yoshi discovers this, Rawk Hawk misinterprets the fake mention as referring to his belt, which in turn hints that he is not a good fighter. Mario is given the belt after defeating Rawk Hawk, however after finding the real Gold Star, he returns the belt.
In Bowser's event after Chapter 6, Bowser and Kammy Koopa find the belt in Rawk Hawk's secret training room, apparently somewhere in the Rogueport Sewers. Bowser believes that it is the real Crystal Star, but it is smashed after Rawk Hawk attempts to attack Bowser and Kammy.
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