Steven Universe: Unleash the Light

You will start the game with Amethyst as a playable character, but you won't have access to Peridot until midway through the third world - specifically during the Be There Or Be Square subquest.   As the trophy states, complete 10 battles with both of them in your party - they must be alive at the end of each battle.  Both have pretty weak defense and HP stats, so I recommend going back to the easy early levels to knock this one out.