Go to the alley right next to Toad's house in Dry Dry Outpost. Jump forward over the left box and the camera will swing around to reveal a hidden area. Talk to Merlee to learn about her special spells. Depending on how much money you spend, she will help you during and after battles. The spells do wear off after a certain amount of time, but until then, it provides you with additional attack power during some battles and more coins after battles.