Jashin-chan is a demon from Hell who serves as the central figure of her story. She is a lamia, a creature with the upper body of a youthful, blonde-haired woman and the lower body of a large, green snake. Her appearance is completed by her icy blue eyes and the two blue ribbons she wears in her hair. Despite living on Earth, her origins are in the demon realm, where she comes from a noble, wealthy family with a name so long it would take thirty minutes to recite in full. Her true age is considerable, as she was alive as a child during the Great Flood, an event that took place over seven thousand years before the events of the series.

Jashin-chan's personality is defined by her selfishness, irritability, and overwhelming ego. She is impulsive, loud-mouthed, and often acts without considering the consequences of her actions. Her schemes are typically driven by a desire for personal gain, such as gambling on pachinko and horse races, for which she has a notorious and ruinous addiction. She can be sadistic, taking pleasure in envisioning elaborate scenarios where she finally achieves her goals. However, beneath her abrasive and self-serving surface, she occasionally displays a reluctant capacity for care and loyalty, particularly toward those she considers friends, even if she would never openly admit it.

Her primary motivation is a desperate, singular desire to return to Hell. This goal is directly tied to her relationship with Yurine Hanazono, a gothic-looking human who summoned her to Earth. The grimoire Yurine used for the summoning did not contain the spell needed to send Jashin-chan back. The only other known way for Jashin-chan to return to her home realm is for Yurine to die. This has led Jashin-chan to constantly and obsessively plot to kill her summoner, viewing her as the shackle binding her to the human world. This overarching goal fuels much of the conflict in her daily life, as she perpetually devises new traps and attacks, all of which invariably fail.

In the story, Jashin-chan is trapped in a chaotic cohabitation with Yurine. Because her repeated attempts on Yurine's life are always met with immediate and brutal retaliation, she finds herself serving as a live-in maid of sorts, forced to do chores and cooking around the apartment in exchange for being allowed to live there. Her role is that of an instigator, whose violent schemes and selfish antics drive the series' continuous stream of absurdist and slapstick comedy.

Her most significant relationship is with Yurine, a dynamic built on mutual antagonism and enforced proximity. Yurine possesses formidable physical strength and a sadistic streak, and she punishes Jashin-chan's murder attempts with creative, gruesome, and often fatal acts of violence, such as dismembering her or using her body parts in cooking. Despite this, they have settled into an odd routine, and there are rare moments that suggest a complicated, unspoken care for one another beneath the relentless cycle of violence. In contrast, her relationship with Medusa, a fellow demon, is one of genuine, if often lopsided, friendship. Jashin-chan relies heavily on Medusa, who functions as her personal source of money and a partner in crime for purchasing weapons, even openly admitting she would not function properly without her. She displays intense jealousy and protectiveness toward Medusa, breaking down at the thought of losing her. Her other key relationships include those with the fallen angel Pekola, whom she bullies, and the angel Poporon, with whom she has a more competitive rivalry.

While not a character who undergoes a dramatic, linear transformation, hints of development are present in the bonds she forms. Over time, it becomes clear that despite her constant declarations of hatred for Yurine, Jashin-chan has come to depend on the life she has in Akihabara and the people in it. Flashbacks show her childhood in Hell with Medusa and another friend named Minos, providing context for her deep-seated fear of loneliness and her desperate need for connection, which she masks with arrogance and cruelty. She can be moved to genuine action by her affections, such as when she becomes enraged and seeks revenge after seeing Medusa cry.

Jashin-chan possesses a remarkable set of abilities, the most crucial of which is her incredibly powerful regeneration. She can recover from being turned into mincemeat, reduced to a pool of blood, cooked into a dish, or even transformed into salt. This ability is often played for gags, with her reconstituting herself after the length of a television commercial break. This power means that Yurine's brutal punishments, while painful and humiliating, are never fatal. Her signature offensive technique is her dropkick, which can vary in power. Her normal dropkick is a wall-level attack, but with the use of a special item called Satan's Tears that grants her human legs, the force increases dramatically. Her ultimate dropkick is an extraordinarily powerful attack capable of propelling her from space and causing building-level destruction. Aside from combat, Jashin-chan has a surprising number of practical skills. She is an excellent cook, capable of preparing a variety of dishes, and is talented at sewing and crafting. She frequently breaks the fourth wall, directly addressing the audience and making jokes about being in an anime or manga. The limits of her abilities are her own impulsiveness and arrogance, which lead her to pick fights with beings far more powerful than herself, as well as a stated weakness that she could be permanently killed by a bullet through the head.