So.... he was Satan? (Samurai Champloo)

Yesterday i finished reading the second volume of Samurai Champloo. It was good but not as good as the the first.

Anyway, after reading and the bogus-bonus volume i got a little confused. I would appreciate it if someone could tell me if Mephistopheles was actually Satan or just a demon. It's quite annoying that i don't know. Also if this bonus story has anything to do with the true story and if it will be continued. I know it says "Will NOT be continued... " but it was the most interesting part of this volume and i wonder what happened.