The trial character gets an additional slot in your pre-existing team. You cannot interact with NPCs, do quests, run domains or co-op. You can fight bosses (cannot collect rewards), mobs and test out how the character works with your team and the open-world.
I would have pulled on Yoimiya's banner if I knew Sayu could ice bridge. Or spent money on Kazuha if I knew his E could be used while gliding. Well, mhy will never actually implement this but there's always some hope lingering around