This is copied over from my TrueAchievements (Xbox) solution. Yes, I have this achievement on my Xbox account.
This achievement requires the hero to ally with all the possible NPCs. The NPCs required for this achievement include:
Yohei: Mission 03 Lord Hadears
Cecilia: Quest 05 Fire Starter
Richard & Robin: Quest 06 Through the Forest
Barbara: Quest 08 Infinite Desert; Quest 26 Isolated Castle (only one required)
Donovan: Quest 11 Bandit Valley
Elena: Quest 14 Nun and the Devil
The Devil: Quest 14 Nun and the Devil
Hank: Quest 14 Battle Ocean
Rex: Quest 15 Lizardman Nation
Natalie: Quest 16 Flower in the Dusk
Dain: Quest 16 Mummy Desert
Karen: Quest 17 To the Snowy Country
Sebastian: Quest 22 Giant Evil Lord
Scorpio: Quest 25 Treasure Land
Catherine: Quest 25 Swamp and Witch
Quest 31 Last Battle, Quest 26 The Return of Hol, and Quest 20 Great Plains do not count towards the achievement.
If you have any ideas on how I can improve this solution, I'd love to hear them! There may be something that isn't here that should be, or an easier way of doing this. I'm always open to suggestions. If you notice any typos or poorly phrased sentences, please let me know.
I'd like to thank AkkonShinigami (Xbox GamerTag) for confirming that Quest 20 Great plains doesn't count towards the achievement JoeDowns68 (Xbox GamerTag) for confirming you only need to ally with Barbara once in order to get the achievement.