Quick Answer
Clear the Platina Special Demon Live quests before you defeat Demon King Platina Goldcaster. If you missed them, revive her at the altar and finish the quests.
What quests can you miss before Platina?
Defeating Platina too fast makes you miss quests. After you beat one of her generals, you get a quest named "Platina Special Demon Live Feat." followed by the general's name.
Clear these quests before you fight Platina if you want to finish every quest.
How do you revive Platina?
Go back to her boss arena, pick up the altar recipe, and use the altar. You can then finish the three missed quests.
Return to the boss arena
Enter Platina's boss arena again.
Pick up the altar recipe
Take the altar recipe from the arena.
Use the altar
Interact with the altar to bring her back.
Start the quests from the log
Start the quests manually from your quest log while you are in the village. They will not pop up on their own.
What happens when she is revived?
She enters the EX phase, which means harder invasions. You can then do three encore quests for the trophy items.
Each revival gives you an Orb of Acclaim from the boss remnants, and the live level goes up by 1.
FAQ
Yes. Revive her at the altar and start the quests from your quest log.
From the boss remnants each time you revive Platina.
It makes the invasions harder, and unlocks three encores for trophy items.
Next, see Warota bosses or the Warota game guide.