One of the cool features of World of Final Fantasy is the ability to summon Final Fantasy characters like the Warrior of Light, Tidus, or even Cloud.
Summoning a Final Fantasy character in World of Final Fantasy is called “Invoking the Champions”. As you meet Champions in Grymoire like the Warrior of Light, you will be able to tackle their personal quests through the NPC called “The Girl Who Forgot Her Name” which is located in “The Girl’s Team Room” that gets unlocked in Nine Woods Tail after finishing the first few chapters of the game. Through her, you can do the Champion’s personal quest by “intervening”. Upon choosing the appropriate quest, for example with The Warrior of Light, you will go through a series of cutscenes involving him and at the end, you’ll get to fight the enemy he’s fighting. Upon the defeat of the enemy, the Champion will be available to summon.
You will receive a Medal for every Champion you “intervene”. With the medal that you receive, you can equip it through the Champion Medals in the main menu. Please note that you can only equip up to three medals at a time.
Once they are equipped, you can then use them in battle depending if you manage to fill up the appropriate gauge while in battle.