Chet Rippo appears in both Paper Mario and Paper Mario: The Thousand-Year Door. In the former, he is found near the port in Toad Town. In the latter, he is located at the entrance of Rougeport Sewers, in a pipe that is accessible only after Bobbery joins Mario.
Chet Rippo from Paper Mario.
By paying him 39 Coins, he can readjust the ranks of the party members (as long as they are not all at their max rank all) or readjust any of your HP, FP or BP, which could be useful to change strategies later on the game.