Weirns

Weirns are a specific type of witch characterized by strong magical abilities and a being or familiar called an astral.

Astrals
The astral seems to be the defining characteristic that separates Weirns from regular witches. Astrals are the constant companions of weirns and are technically the embodiment of the weirn's power. They can be used to increase a person's power or give them inhuman powers, such as flight. Astrals seem to be a floating mask with black and white smoke coming all around it. Each astral has a different pattern on its head.

Astrals are probably mute, as Alex Treveney's Astral could not tell her about the Sohrem inside her, instead pointing to its arm (which it had turned into the cloak that surrounded her), but she could not understand.

The only way for a weirn to control their astral is by bonding with it. This is shown in the second volume, while Alex is thrown out a building for her final test, and she has a flashback to when she complained to Sarah Treveney about her astral not following her commands, and Sarah tells her to bond with the astral.