Mabriella du Feza is a high-ranking member of the Wrights of the Seventh Sun, and their liason within the Glim Macula.
She displays some sort of supernatural power involving the hair of some creature, able to penetrate people's brains and read their minds, and control puppet versions of herself made out of hair. This is self-evidently flouting the Macula's own beliefs around heartsblood creatures, as people with power often do.
Blick finds her extremely creepy, and is willing to sell her out.
When Chine was captured after attempting to kill an interviewer, they woke up in du Feza's clutches in a throne room of the Interred Citadel. She tortured him both physically and mentally, but was unable to gain much useful information. She also took his egg sac, and used it to further power an orrery mimicking the form of Zevunzolia.
When members of the Blackwick Group broke in to rescue Chine, they confronted du Feza. Pickman and Es were able to deal a significant amount of damage to her two puppet bodies (one dressed as an equestrian, the other in a fancy ballgown), but she escaped the Citadel as it collapsed, leaving behind crumpled wicker.