The twist became obvious the moment she agreed to spend the night in a fucking dungeon that is supposed to be full of monsters despite not knowing if MC would be powerfull enough to protect her. Because that meant that she was either strong enough or confident enough to take care of the monsters herself.
Ah yes. What a great master trapper he is. He immediately fell for the honeytrap, the most ancient and basic of human traps.
I'm starting to feel sorry for the demon princess. She ended up summoning the most incompetent person possible and she's basically stuck with him.