This show is straight 100% total schlock. I mean, describe the series on paper without making it sound stupid. It's impossible:

"There's this guy who is a (kinda) half-oni pitfighter working for a carnival in a Japanese slum who meets an immortal (kinda) 14 year old girl, who is actually 900 years old. She has her head in a birdcage and is carried everywhere by her #battlemaid. The half-oni pitfighter guy fights the #battlemaid and wins because he's the bestest fighter ever. The woman who is just a head now was attacked by a different half-oni because half-oni are special (kinda) in that they make it so that when they hurt you wounds take longer to heal, which (somehow) translates into her magical regenerative powers not working and causing her to not regrow her body (which they stole and left her head behind because...why not?), which is something that she, as the world's only immortal, can just do (I guess). The head and the pitfighter guy make a deal to travel to Europe (which is thankfully very small, as well as geographically, culturally, and linguistically homogeneous and an easy place to find one specific person) to try and find the person who (conveniently) turned him into a half-oni and who stole her body (for some reason). Mr. Bodysnatcher has an M on his cane and totally won't turn out to be Moriarty from Sherlock Holmes, because that'd be totally stupid for a detective mystery battle shonen to asspull Moriarty as its main villain. In order to prevent the pitfighter guy from going full murder hobo because of his powers (for some reason), the pitfighter has to totally make out with the 14 year old girl who is a 900 year old decapitated head now on the reg (yes, it's totally essential to the plot). Also, after they find John M-Cane, the decapitated woman wants pitfighter to kill her for reasons that the show hasn't bothered to go into yet and which the pitfighter never asked about. Also being a half-oni makes the pitfighter have terrible physical deformities, such as a set of very sick looking full body tattoos and literally nothing else like horns or a barbed dick (as far as we know)."