Lisara Restall, heiress of the noble Restall family and daughter of Almeia Restall, is an elite Grim Reaper from the parallel dimension Grimwald. Her mission sends her to the human world to locate the magically potent "The One," crucial for averting crises threatening her homeland. This quest leads her to forge a temporary spirit contract with high school student Ryosuke Kaga, utilizing the hilt of the broken legendary sword Gram. The contract enables Lisara to draw energy from Ryosuke, sustaining her powers and physical form on Earth, but initially imposes a three-month lifespan limit on him due to the energy drain.

Physically, Lisara possesses flame-red hair reaching her hips, matching red eyes, and a slender figure. She alternates between a standard school uniform and attire consisting of a collared, sleeveless white shirt, black skirt, and thigh-high stockings. In combat, she transforms into a form-fitting black robe with a floor-length cape, thigh-high boots, elbow-length gloves, and head-wings. This battle attire, sustained by her energy reserves, deteriorates upon damage or depletion and restores when her power replenishes.

Her personality embodies the classic tsundere archetype: outwardly cocky, condescending, quick-tempered, and aggressive, yet harboring a strong sense of responsibility, loyalty, and compassion. She frequently reacts with violence or blushing to Ryosuke's lecherous behavior, though she gradually develops genuine romantic feelings for him. Pride and situational embarrassment prevent her from expressing these feelings for much of the series, leading her to rationalize their contract as purely transactional. Jealousy surfaces when Ryosuke interacts with other girls, particularly childhood acquaintance and rival Iria Fukumune, or her distant cousin Quele Sellier, whom she mentored and who addresses her as "Onee-sama."

Lisara wields the scythe Carnun Pladur, channeling fire-based abilities for combat, including flame-enhanced strikes and spells. Her battle effectiveness depends directly on Ryosuke's energy, originating uniquely from his libido. This dynamic forces her into compromising situations where physical proximity is necessary for energy absorption, sometimes causing unintended arousal or embarrassment. As their relationship evolves, Ryosuke's focus shifts exclusively to her for energy generation, heightening their emotional and tactical synchronization.

Her character development focuses on overcoming emotional barriers. Initially prioritizing her mission over personal connections, she attempts to transfer Ryosuke's contract to her friend Mina Okura upon learning Mina might be "The One," believing this would ensure his survival. Pivotal moments—Ryosuke's near-death experiences, his self-blame for catastrophic events after a battle, and his eventual death—compel her to confront her feelings. In the climax, she separates her soul from her body to retrieve Ryosuke's spirit from the afterlife, confessing her love to persuade him to return. This act solidifies their romantic relationship, though she continues to react tsunderishly to his advances afterward.

Additional elements include her March 8 birthdate, upbringing alongside Iria, and role in conflicts involving antagonists like Gardarbolg, who seeks to dismantle the barrier between Grimwald and the human world. Familial ties, particularly with her mother Almeia and the Restall household maids Hild and Urus, influence her noble demeanor and sense of duty. Expanded media, such as OVAs, feature scenarios like a school swimsuit contest, providing further comedic and situational development through her interactions.