Board Thread:Questions and Answers/@comment-2.98.58.229-20160215175235/@comment-25942629-20160221211147

The demon forms a second personality of that person. In the Catastrophe LNs, you can see the two Mahiru's go head-to-head as Mahiru-no-yo tries to lure Guren into become a demon while Mahiru fights against her with all of her strength to protect Guren. The demonic personality appears to eventually overcome the human part until the human part no longer has any say or has any control over the body. As Mahiru and Mahiru-no-yo struggle for control, she begins to spend less time as a human and more time as a demon.

Considering how Guren tells Shinoa that Mahiru is dead in Vampire Reign, the original personality appears to be consumed by the demonic one and disappear entirely.

The demon is born based on the desires of the human it sprouts in, so the original personality influences it in that regard. It's much nastier, though, without any other sympathies or kindness. Mahiru-no-yo is influenced by Mahiru's longing and love for Guren and uses that to begin turning him into a demon and bring him down with her instead of using those feelings to protect and defend him like the original Mahiru would.