Pest, alias Black Percher, is the Demon Lord embodying the Black Plague. She manifests as the collective incarnation of eighty million spirits who died during the pandemic. Her origins lie in a human life where she contracted the plague and was imprisoned by her fearful father. As the disease consumed her family and community, her final curse granted initial spiritual power, binding the souls of plague victims to her as she wandered for centuries. This accumulation led to her summoning by the Demon Lord alliance Ouroboros in Little Garden, where she gained full consciousness and control.

Initially leading Grimm Grimoire Hamelin alongside the avatars Ratten and Weser, Pest orchestrated a Gift Game targeting Salamandra to compel powerful groups into her alliance. Her primary objective was revenge against Shiroyasha, whom she blamed for the plague's suffering due to Shiroyasha's sun association. During the conflict, Pest deployed her Gift "Black Death," releasing lethal black winds fatal upon inhalation. This ability embodied the plague's virulence but held a weakness to sunlight. Her plan failed when defeated by the combined forces of the No Name community and allies, destroying her physical form.

The Gift Game rules enforced her subordination to No Name. Her soul was reconstructed and bound to serve the community, initially as a maid alongside the vampire Leticia. Initially resentful and prideful, Pest openly insulted No Name members but gradually developed respect, particularly for Jin Russell. A pivotal shift occurred when Jin promised to help fulfill her wish—presumably related to the eighty million souls—during their imprisonment in Sandora's castle. This pledge solidified her loyalty to Jin, transitioning her into a dedicated protector of No Name.

Pest retains her abilities post-defeat, though diminished from her deity-level power during the Gift Game. She can still unleash the Black Death winds, swiftly eliminating threats like invading Titans. Her personality evolved from detached arrogance to displaying concern for others, mourning Ratten and Weser's sacrifices and internally accepting Leticia's camaraderie. Despite lingering pride and a quick temper, her integration into No Name alleviated her centuries-long loneliness.

Physically, Pest appears as a pale girl with purple or red hair tied back by mouse-eared black bows, red eyes, and a petite stature. She typically wears Gothic-style dresses with frilled accents, thigh-high stockings, and a blue horn at her waist. In her No Name role, she dons a standard maid uniform. Her journey continues in the sequel *Last Embryo*, where she remains under Jin's command as part of the rebuilt community.