Bacillus is a parasitic alien criminal who partners with Geega, serving as an antagonist across multiple narrative continuities. This slime-like entity assimilates and controls living hosts, cycling through various animals and humans before physiological incompatibility causes their bodies to decay. This cycle forces him to seek new hosts frequently, a process termed Possession Burnout.

In the original manga and OVA, Bacillus acts as a secondary threat after Geega's defeat, showing limited cunning and combat ability. The remake manga significantly expands his role, portraying him as a more intelligent, tenacious, and malicious Arc Villain. This version commits premeditated cruelty, murdering detective Goro Nagase to steal his identity and subsequently butchering Nagase's entire suspicious police team. His actions escalate to indiscriminate violence, including a school shooting where he targets civilians and children to draw out Birdy.

Bacillus's psychological state deteriorates across all media, with insanity intensifying after each host switch. The remake manga depicts calculated malevolence early on, such as blackmailing fellow criminal Gomez by threatening his family. However, his mental stability erodes into full Ax-Crazy behavior, culminating in frenzied rampages. Conversely, the *Decode* anime continuity introduces him as already animalistic and devoid of strategic thinking.

His demise varies by adaptation. Gomez or the cyborg Salamander incinerates him in the original and rebooted manga after deeming him uncontrollable, triggering a Villainous Breakdown where Bacillus screams impotently while burning. In the OVA and *Decode*, Birdy kills him directly during confrontations. Bacillus's parasitic abilities and violent sprees consistently position him as a Knight of Cerebus, marking a tonal shift toward darker narratives. His mucus-based biology creates a logical weakness to fire, which proves instantly fatal when exploited.