I never played Demon Souls but beat the main game and all the DLC for Dark Souls 3 numerous times. Are the systems in demon souls pretty much the exact same as Dark Souls?
It's similar to 3 in structure but in feel it's closes to the first Dark Souls in feel and atmosphere.
Playing any of the souls games you will make it feel pretty familiar but the world tendency system and the upgrade path for weapons is definitely in a prototype stage in that game (farming for blade stones being a big pain in the ass, and side quests will only appear depending on how light or dark the given level you are going through is set at.)
Without a guide you will probably run into different things every playthrough based on your play.