Callisto, known as Keto, serves as a lieutenant within the antagonistic Dark Joker, a group dedicated to eradicating universal life. Under commander Bross, he spearheads invasions targeting Earth and Queen Earth, wielding the corrosive Black Vaccine to unleash widespread corruption. Keto’s volatile rivalry with comrade Dewey erupts into violent clashes, fueled by their struggle for leadership approval and opposing strategies, fracturing Dark Joker’s unity and hastening its downfall.

Keto directly orchestrates the mortal wounding of Kanon, a pivotal ally to the protagonists, delivering injuries that provoke Kanon’s rejection of healing. This act becomes a catalyst, transferring Earth’s defense to others. Later, Bross betrays Keto, abandoning him as disposable—a testament to Dark Joker’s merciless power structure. Keto’s ultimate fate post-betrayal remains undocumented in primary series lore.