Everything is registered at the beginning! But for Berries or one-use items (Focus Sash, White Herb, etc), the character can replace the used item with another item of the same kind between battles. (So, no switching a Persim Berry for a Rawst Berry, for example, but you can give them a new Persim Berry if they consumed it in the previous battle.)

Permanent items like Quick Claw or Charcoal cannot be swapped out.

Choose wisely!