He wants to reverse isekai her back to his world. So he can beat somebody there. Now that's kinda interesting.
But I could've sworn he didn't have that reason back when he was first stranded in this world...
I'm somewhat surprised Mersha readily agreed to leave her own world. Then again, being the leader of a declining kingdom in a crappy, oppressive world that's hostile to her kind could be reason enough. Maybe she's tired of such a stressful role and feels like she isn't cut out for it, since she really only has the throne because she happens to be the strongest among her kind.