Board Thread:Questions and Answers/@comment-2.98.58.229-20160215175235/@comment-25942629-20160229153536

Aiken Pandora pretty much says it all. There has been no mention of a second demonic personality growing inside Yu, Yoichi, or Kimizuki yet in the manga. The curse restraining the demon's power and influence was developed by the Order of the Imperial Demons after Guren had already been using a demon weapon and had turned into a demonic state very similar to Yu's current state. It took quite a bit of effort to restrain Guren and then put these curses on him. After that, the Order of the Imperial Demons, which later becomes the Japanese Imperial Demon Army, uses these curses to manufacture cursed gear. Yu's demon was already restrained by the time he got it, so there was no sign of a demon growing inside him until now. However, by stabbing himself through the chest and having Asuramaru drink all of that blood, the curse has since been removed. He and Asuramaru now have an unrestrained and unlimited bond like Guren had with the demons he first came into contact with.

In the first tankoban, which is out for sale now, you can read about how the demon!Guren introduces himself to human!Guren. It's beautifully chilling.

You can tell the curse has been removed because Yu no longer has curse marks on his face in that form. When he partially became a demon while trying to rescue Guren from Crowley, the curse marks were still present.

Considering all that, Yu probably has a demon!Yu growing inside him by this point and has become a namanari, but future chapters will explain that for certain.