Radiant Historia/WMG

Teo and Lippti
Aht She is also the only party member besides Stocke who can see him interacting with them before the last chapters.
 * It's revealed that they're

Party Member levels and such carry over because of Stocke's bond with them.
The purpose of the White Chronicle is to prepare the Sacrifice for the Ritual by helping them form and strengthen their bonds with others, right? Well, in order to strengthen any bonds those others would need to remain alive, and would require the skills needed as such. Every time Stocke uses the Chronicle, it takes a "snapshot" of the rest of the party as perceived by Stocke, and each time he travels to a node it reapplies the abilities from the most recent "snapshot" to all of the pertinent characters, advancing them to whatever level of power they were at.
 * This could also explain the effect of feelings and emotions transferring between timelines: as Stocke gets closer to people, he gains more of an understanding of how they think and feel, and how his choices have affected them. These are incorporated into the "snapshots", and are applied when Stocke transfers to a node. This explains why these carryings of emotions and thoughts become more and more pronounced as the game progresses.
 * Following this logic, At Journey's End's requirements actually make sense.