I noticed it early on in this series, but I just felt like commenting on it now. I think this series does a good job of having moments, but is poor at maintaining a consistent world/character arcs. (Plot devices are conveniently introduced, many things are left unexplained, characters' motivations and actions feel at odds at times) What kind of let me down this time was how the young charmling girl (Izumi) addresses the thing coming out of herself as a separate entity, even earlier they note that the girl is a puppet, a persona, put forth by the charmling. All the sudden Izumi-chan is in control? When she's talking to the thing out of her back that would be like a puppeteer using a puppet to talk to themself. You could argue that is what is happening, but based on how the charmling behaved before this seems unlikely. The author is redefining what the charmling actually is.
I love the art and the main relationship, but the capricious developments and lack of explanation within the story holds this series back from being truly great, rather than just good.