Without reading ahead and spoiling anything at all, here are my two thoughts at this point:
① Now we all have much better understanding of why so many of dungeon's visitors seem to “lose heart” as Ageha said. They could have all freed her in the previous loops successfully, then triggered a suicide of her true fragile self, then met her other murderous self on the next loop. Add a vampire to that mess and then a man's heart can truly break, especially if one does not have any tragic emotional baggage from the past (like of our main character) and thus is free to love Ageha and then be devastated both by her suicide and by the subsequent personality changes. And naturally Ageha does not remember any of that. Either she is reset with everyting else and thus she sees only the last loop (seems the most plausible) or her memories are suppressed by her condition (most likely a dissociative identity disorder).
② If our main character had some genre awareness (but that would require the impossibility of him being an otherworlder like Ageha), he could “double his stakes” by telling Ageha's other self that he fell in love with her own twisted personality instead of Ageha's true fragile self.