Note: Earn the other princess endings before this one.
Complete the game with all princesses happy. From the throne you can check the citizen list which shows a face next to the princesses. You need to make them all like you. You do this by spending time with them, while playing the game always take a princess and try not to get her killed. After a few hours together check the citizen list, if they are happy switch to the next one.
This trophy is the biggest grind of the game, you can pick a princess and sit idle in the courtyard and wait for it to go up and use moody incense items to help.
Once all princesses have a smiley face, kick your princess from following you, get your army together and speak to Azul in the courtyard to trigger the ending, pick any princess from the options and defeat the final boss for the trophy.