Demon Slayer: Before I Transmigrated, I Was Just a Blond Punk

I transmigrated, and I’m the eldest son! (Please leave lots of reviews! If this gets popular, I’ll be a veteran author.) 666: a mildly satisfying story + System from the start + pure Japan + immensely popular protagonist + villain (Warring States period). (Not a strictly faithful Demon Slayer world: it’s our world! There are martial arts and a vampire world.) Important: Lots of original settings, lots of original settings!!!! In the vampire world, they are twins, reborn, and after returning, these people don’t want to go back to their own world (they are the protagonists of the other world). So, this is actually about someone’s reincarnation returning to their own era. Michikatsu is the original Tsugikuni Michikatsu from the Warring States period, and his personality will change.