If it's a character from a story Gacha they'll be added into the main Gacha, if it's a character from a limited Gacha like the MomoHime, or Bremen Band, they will disappear after the time limit. I'm guessing they'll have them again, if not they're kinda screwing themselves, being not everyone has the cash to waste on the game..
I'm wondering about the new boss, do you have to use up all purchased attacks before reset, or do they carry over?