Gold, ISO-8, and Catalysts
Gold, ISO-8 and Catalysts actually work together to improve your Champion's abilities. You need a certain quantity of ISO-8 to gain one level each time, and then you need Gold to actually infuse it into your character. After hitting max level, you can use Catalysts to increase your character’s rank unless that’s maxed out too. One and two-star Champions only unlock their later special moves when you increase their rank.