Exp. Booster

From the Super Mario Wiki
The Exp. Booster as it appears in the game

The Exp. Booster is an accessory that can be used by anybody in Super Mario RPG: Legend of the Seven Stars. It doubles the experience that Mario, Princess Toadstool, Bowser, Geno and Mallow get from a battle. It can be bought at Seaside Town from Frogfucius' Student for twenty two Frog Coins. The Exp. Booster can only be used by one party member at a time, as with all other accesories.