Jashin-chan is a lamia summoned from Hell to Earth by college student Yurine Hanazono through a ritual. Trapped without means to return, she relentlessly attempts to murder Yurine, convinced this is her only solution. All assassination efforts fail, often resulting in severe punishments like dismemberment or immolation from which she regenerates completely. Her appearance features a humanoid upper body with blonde hair tied by light blue ribbons, blue eyes, and a green serpentine lower half. Her hair typically covers her unclothed torso, though she dons a white blouse and blue skirt for formal events and a red hooded cape with heart motifs in later seasons.

Personality blends overt selfishness, arrogance, and cruelty with concealed compassion. She exploits her friend Medusa for money, bullies lesser demons like ice devil Koji, and provokes angels such as Pekora by stealing their food. A compulsive gambler, she squanders funds on pachinko. Core traits include gluttony, impulsivity, and explosive rage—especially over wasted food, which triggers violent outbursts. Yet she inconsistently demonstrates loyalty: nursing Yurine during illness, expressing profound guilt after accidentally killing a pet fawn named Azusa II, and protecting Medusa from danger. These acts contradict her self-proclaimed evil nature, leading her to mask kindness with tsundere behavior or claims of selfish intent.

Key relationships define her evolution. Hostile assassination attempts against Yurine shift toward reluctant coexistence, with occasional acknowledgment of mutual dependency. Her bond with Medusa mixes financial exploitation with deep affection; she risks her life for Medusa while using terms like "aishiteru" for platonic or romantic love. Childhood ties include Minos, a minotaur immune to her schemes, and Persephone, a mentor. She initially despises Persephone II (tasked with teaching her the "Refined Dropkick") but develops sisterly regard. Antagonism toward angels Pekora, Poporon (nicknamed "Nikujaga"), and Pino evolves into uneasy alliance after they lose their halos and become stranded on Earth. She forms a reckless friendship with jiangshi demon Kyon-Kyon—dubbed "The Dumb Devil Duo"—despite disapproval from Kyon-Kyon’s sister Ran-Ran.

Abilities extend beyond regeneration to dice manipulation for controlling outcomes, which she dismisses as useless. Unexpectedly, she possesses refined culinary skills, earning praise for baking and cooking despite general incompetence. Hidden talents include singing, an appreciation for classical music like Holst’s "The Planets," and sporadic intellectual curiosity about subjects such as human evolution.

Character development unfolds across the series. Early episodes frame her as a villainous failure with perpetually backfiring schemes. Gradually, she exhibits protectiveness: defending Pekora from divine judgment, rescuing Poporon from online harassment, and aiding Pino during paranoia-induced crises. A pivotal heroic sacrifice sees her dropkicking a falling sun to save Earth at Season X’s conclusion, though her survival remains ambiguous. Core flaws endure, including habitual lying, recurring gambling, and an inability to abandon self-serving instincts.

Background reveals vulnerabilities like childhood loneliness from bullying over her demonic traits and disappointment in her adult form, which fell short of youthful expectations of becoming "colossal and terrifying." Recurring gags feature fourth-wall breaks referencing publishers or censors and police officer Mei Tachibana misidentifying her serpentine body as "Orochimaru."