I forgot which Manhwa this is.
I remember a particular Manhwa I read long ago, a tipical isekai story that has a girl appear as a supporting character to a hero in a game(?), using her outside knowledge to assist the party and try to reach the end of the game.
However, after reaching the 'fake-out' ending that leads to a New Game Plus mechanic for the true ending, the protagonist is left behind by the hero in a cave where he is supposed to save her. Fortunately, the villain of the story, a knight who is fated to become the demon king (the true final boss), saves her.
The reasoning behind him saving her being that he knows something is up and he wants to use her knowledge to stop his slow transformation into the demon king.
I did try to google around and swap wording a little to see if the title was as close to the description as possible but I didn't find it. I would appreciate any recommendations that have a similar story concept or the very same manhwa that I am trying to find!