People's Hero

I feel like it needs to be established that affinity is a stat related to the player's relationship with a certain Reign, not the princess that rules it. Affinity is for the faction, relationship points are for princesses. That being said, you can increase affinity by giving certain gifts to the princess of a given Reign. You can also increase it by completing quests affiliated with that Reign, or aiding the Reign in field battles when they are at war. This achievement basically requires you to max out your affinity with any Reign, so just keep doing stuff to affiliate yourself with them more and you're sure to get it.