To make your heroes and villains more powerful so they move up through the ranks and reach their full potential you need to upgrade them by collecting shards. The rank of a character is determined by the number of stars they have which are obtained by collecting shards. When you have collected the required amount of shards your character will advance to the next rank where their base stats will be increased and new Super Powers that augment their combat capabilities will be unlocked for them. Each Legendary Rank you achieve allows you to permanently enhance one of your abilities.