The moniker Monster-brother was used by Cody Sunn-Childe to describe the physical psychic forms that embodied his intense and aggressive emotions.
To preserve his internal tranquility, he allowed these entities to wander freely within the Kingdom of Dreams's jungles. However, after Chewbacca and Lando Calrissian incited some of these creatures to acts of aggression, Sunn-Childe found it necessary to reintegrate them back into his own being.
Subsequently, Plikk's assault on Captain Cody triggered his anger, which resulted in the monster-brothers manifesting as enormous demons and launching an attack against the Imperial fleet.