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Nothing else. You only "present" an Evil Character one time during the battle. The only way you can bring others in is with a "band" or "add to battle". Additionally, some cards give you the option to present a new blocker in place of the old (e.g. Lion Dwelling with the Calf), and some let the rescuer choose the blocker, and that replaces an old one with a new one.
So, in the case of Darkness above, unless they negate it the rescuer loses the battle?
Only if the card says so.
Yes. That is the language that says you can present a new blocker. Herod's Treachery, which just says you withdraw, does not.