A player collects in-game comic book covers to unlock new characters and improve existing ones.[7] Each character has a set of real comic book covers associated with them, which represent the character's abilities and allow the player to improve the character's abilities or level them up.[8] In July 2014, Team-Ups were introduced, allowing a player to battle with single-use abilities from characters that aren't part of the player's teams.[9] Characters are ranked in different tiers using stars.