Rookie

From the Super Mario Wiki

Rookie is the name Popple calls all his partners during Mario & Luigi: Superstar Saga. Here is a list of characters who have become partners of Popple and been called "Rookie" by Popple:

• Bowser, the first known Rookie who is fought two times in the game.
• Birdo, the second known Rookie who is fought one time in the game.
• Popple, who at the end of the game, became a "Rookie" of a Toad in Little Fungitown.