I cannot, for the life of me, figure out what this guy was in his past life. He says he was a "knight", and he obviously has a lot of experience in the dungeon, but he had no idea who the guildmarm- sorry, guildmaster was so he couldn't have been an adventurer: you'd definitely remember the guildmaster if you've ever seen them even once. So what kind of knight? Lorded knight? Was he in the service of some noble or governmental organization? What was he doing in the dungeon in the first place?
I wonder if we're ever going to get any explanation whatsoever on his past life beyond him constantly talking about how he's a knight, which is a pretty meaningless thing to say without context. Considering the point of this manga, I doubt it. It's pretty apparent you're not supposed to think about that kind of thing.