The Devotee is a failed vessel, but one who had found out about her origins -- and actively attempted to take in the infection after learning of it. she's a coward who refuses to take her own life, which ironically seems to be the main thing keeping her mind from fully submitting to the infection as well.

She loathes the pale king, loathes anyone who would suggest curing her infection, and most of all loathes other vessels. They are mirrors for what she is rebelling against, as well as being in the shape of herself -- her destiny, in the shape of herself.