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Lusus naturae, (shorthand Lusus, plural Lusii, also referred to as custodians), are animals who look after young trolls, fulfilling the role of a typical parental figure. Lusii are either naturally occurring, or artificially reduplicated by Corporate. Artificial lusii often take on more of the traditional caretaker role, while a relationship with a natural lusus tends to be more symbiotic in nature.
Lusii are naturally blackblooded, changing to match the blood color of their charge after a bond is formed. A lusus' eyes fill in with this color as well.
Lusii in Vast Error
Sova's lusus. An owl-like creature that Sova has as a custodian. She doesn't live inside of Sova's hive due to her size.
Dismas' lusus. An abusive Skulltitan lusus whom Dismas frequently gets into fights with. This lusus is strong enough to leave notable scars on his charge. He kidnapped Dismas when he was young, and forced him to mine for radioactive ore to feed him.
Jentha's lusus. A large caterpillar-like creature that is exceptionally long.
They spun themself a cocoon shortly after meeting Jentha, and have not emerged since.
Presumably alive since Zekura was, as they were in the shadows while The Executive was walking through his hometown.
After fulfilling their promise to Cyprim -- deceiving Albion in order to unlock her rage -- they faded away entirely.
Serpaz's lusus. A Compecado lusus with the ability to suck negative emotions out of Serpaz.
Laivan's lusus. The only artificial lusus out of the main cast. Entered the Medium with his charge.
Taz's lusus. The two seem to have a somewhat testy relationship.
She was devoured by the mutated Lusii of the Black Depths, revealing Taz's entry codes.
Murrit's lusus. He dies for an unknown reason.
It is implied that Calder first bonded with a male anglerfish lusus; when it went on to mate, it attached itself to the body of a female anglerfish lusus, as per the parabiotic reproductive process of some anglerfish species. The female anglerfish now acts as Calder's surrogate lusus. 
Hummingbird Moth Lusus
Secily's lusus. YVES' design is based on them.
Yeshin's lusus. They are implied to have an abusive relationship with their charge, since they are responsible for Yeshin's limp.
Lutzia's Lusus. They spend most of Lutzia's earlier life in a larval state, only pupating when Lutzia is about to enter adulthood. After their pupation, they are tall and strong enough to allow Lutzia to fly with them.