She was never his girlfriend.. and the plot didn't  "thicken" we in fact know nothing of the plot.. the story just had randomly introduced how the supposed villain is all about rei, while to the reader he was just some random healer that was abandoned in a dungeon and thought dead 😐
Story has yet to explain anything, so far it introduced things like the random demon operation that popped up, or the slave trader that out of nowhere just gave him information on the person he was looking for, but the chapters feel kinda jumpy in things happening like going from one event to the next