This achievement takes the longest to achieve, and should be left for last. This achievement requires you to:
1. Talk to all characters when they have a ,(...) over their heads enough for 500 souls. This includes the dragon who is in the 6th area, after you beat the boss there. The fastest way to get his souls is to lock the dungeon and teleport to him, then retire, and repeat. Pet him while you're at it for another achievement and for a cute animation.
2. Get the achievements Erebus Purified (complete all scar challenges) and Just the Beginning (unlock Soul Shop Overload).
3. Find all 4 eggplants and interact (rb) with them. This requires either flight from the House Rules or Icarus Wings. One is at the top left of the tutorial area. Another is at the top of the area with the gold chests in the red portals. Third is in the snow area with the lake above the rock between the start of the room and the statue that gives you permanent powerups after a trial. Fourth is to the left of the boss entrance for the Sun Tower.
4. Get the True Ending, by killing all prime bosses in a single run, which are unlocked by getting to NG+7. You also need to get all of The Traitors pre-battle dialogue by beating him 8 times. You enable the options to fight these bosses through the Threads of Fate at the red lady. You know you have it if the dragon shows up.
If all characters have hearts over their heads in the ending area, you've got it. Also talk to the dragon here for the Had to Do It achievement if you haven't already.