Bem, here's a spoiler for you.

She was only partially possessed by a demon. Camus is a powerful mage, so the demon cannot fully take over her body. When the demon does seize control, Camus's soul prevents complete possession and instead forms a contract. The agreement allows the demon to control her body for half a day, after which Camus regains control. Though Vikir will be able to pierce someone's soul because of his sword, it is also the reason why Vikir can separate the demon from Camus's body in the later chapters.