Look-Alikes and Counterparts

After reincarnation, miraculously, everyone ends up being reborn in the same area. Only a few of these individuals remember their past life.
Eren Jaeger, the now oldest of the group, decides he wants to turn their past lives into a story in order for everyone who doesn't remember to understand, and to pass on his story on to his children.
Unfortunately there seems to be some complications with everyone's memories, and Hanji's solution seems to mean more trouble than the whole affair is worth.