Board Thread:Questions and Answers/@comment-25942629-20160528154941/@comment-25942629-20160530103027

A human who becomes a vampire goes through some extreme personality changes. Although Yu has become a namanari like her, his demonic personality appears to be far more immature and less intelligent than hers. He loses his temper easily when he cannot get his way, and his behavior makes it very obvious that the demon is in control. Mahiru's other personality was far more cunning, and no one else realized she had two of them until it was already late.

If Yu becomes a vampire, I think he will undergo the standard vampire personality changes. There is a good chance his personality will become more like Mika's rather than more like his demon's.