Character recruitment in The Last Promise's post-game area, the Constellation, works quite simply. Anakin and Shon join automatically, and almost all other characters, including characters that are originally enemies in the main game, join after you defeat them in their respective Stars.

You can re-recruit characters that have died or been removed from the party via for the vortex by defeating them in their Star again. Note that Anakin can't be lost or revived. Also beware of losing special items that can't be regained.

There is one more exception, which is that Noah has a 1% chance of joining on the Constellation map during any turn. He can join anytime after the first time you enter the Constellation. He rejoins through this same condition.