Cerebrum is a shapeshifting antagonist who exploits negative emotions, offering Diablos Tarot cards that transform victims into Daemonia monsters so he can harvest their souls. He lacks a fixed form or gender, capable of altering his DNA to mimic any appearance; his default guise manifests as a teenage boy with light blue hair obscuring one eye, distinctive bright blue eyes, and a black cloak bearing blue patterns. His powers include weaving potent illusions, such as trapping targets in temporal loops to endlessly relive traumatic events.

He operates with cold cunning, deriving entertainment and strategic advantage from despair. This sadistic streak drives him to corrupt individuals and turn allies against each other, exemplified when he impersonates Akari Taiyo to manipulate Luna Tsukuyomi, forcibly implanting a Diabolic Tarot card that transforms her into a demonic wolf hybrid. Cerebrum harbors profound disdain for humanity, viewing humans as "disgustingly idealistic" and inherently flawed, fueling his mission to eradicate their optimism. Paradoxically, he demonstrates protective concern for the Daemonia's future, seeking to secure independent forms for his species free from human hosts.

His backstory suggests a connection to Akari Taiyo's lineage. Episodes imply a possible relation to her mother, Hinata Taiyo, through a vision during his defeat. The light novel further suggests Akari's father, Hajime Takatori, was a Daemonia researcher infected during her conception, hinting at Cerebrum's potential biological or symbolic role in her hybrid heritage. This culminates in his proposition that Akari bear his offspring to propagate a new Daemonia generation—a plan that ultimately fails.

Cerebrum collaborates with the corrupt politician Den'ichirō Hakama, eliminating Hakama's rivals to advance the politician's ambitions. His schemes encompass orchestrating mob attacks on Sefiro Fiore's headquarters and psychologically tormenting magical girls, pushing them toward despair to create more Daemonia. His final confrontation with Akari involves revealing her half-Daemonia nature and her father's infection before he is defeated.