Mirajane Strauss faced profound loss early when her parents died, compelling her to shoulder responsibility for younger siblings Elfman and Lisanna. At thirteen, seeking to aid townsfolk, she confronted a demon within a local church. Unaware of her latent magic, she unintentionally absorbed traits from the demon during the encounter. Fear branded her a monster, driving the siblings from their home. They found sanctuary in the Fairy Tail guild, where Master Makarov identified her transformation ability as Take Over magic, not demonic possession. Despite this explanation, Mirajane grappled with persistent self-doubt about her humanity until Elfman and Lisanna mastered their own forms of Take Over magic, proving she wasn't alone.

Her youth earned her the moniker "The Demon," a reflection of her formidable power, aggressive nature, and punk-gothic style. Fierce clashes with Erza Scarlet defined a potent rivalry in the guild's early years. Her notoriety spread beyond Fairy Tail, making her infamous among criminals and rival magical organizations, showcasing her combat-centric mindset and intimidating aura.

A critical mission with Elfman and Lisanna became a turning point. Elfman attempted a risky Full Body Take Over of a powerful beast, lost control, and violently threw Lisanna, who was subsequently presumed dead. This tragedy shattered Mirajane emotionally. She lost most of her magical power, abandoned combat, and underwent a complete personality shift. The once-temperamental warrior transformed into a gentle, nurturing presence within the guild, assuming roles like head waitress and modeling for Sorcerer Magazine. Her new demeanor mirrored Lisanna's kindness, though deep emotional scars remained, causing her to avoid using combat magic for years.

Her primary magic is Take Over, specializing in the signature spell Satan Soul. This allows her to assume the forms, abilities, and powers of demons she "knows." Initial use after her long hiatus caused massive magical explosions due to pent-up power. Early limitations imposed extreme exhaustion, restricting her to one full transformation daily. Intensive training later enabled multiple transformations with reduced fatigue. Key Satan Soul variations include her Base Form, granting enhanced strength, speed, durability, poison resistance, flight via bat-like wings, and darkness magic for energy blasts or shields; Sitri, a high-tier form noted for city-level destructive capability used decisively against opponents like Jenny Realight; Seilah, acquired after defeating the demon Seilah during the Tartaros arc, granting access to the Macro curse for mind control and battlefield manipulation; and Algeria, her strongest known form, capable of defeating multiple powerful enemies simultaneously during the Alvarez Empire arc. Additional skills include Transformation Magic, used to impersonate Lucy Heartfilia during the Phantom Lord crisis, Sleep Magic, and Water Magic.

Mirajane shares an intensely close bond with Elfman and Lisanna, prioritizing their safety above all. Elfman's near-death at Freed Justine's hands during the Battle of Fairy Tail arc triggered her combat return, unleashing Satan Soul to protect him. The emotional reunion with Lisanna, discovered alive in Edolas years after her presumed death, restored significant joy and stability to the family. Within the guild, she maintains a motherly relationship with members like Natsu Dragneel, whom she comforted in childhood. Her dynamic with Erza evolved from rivalry to mutual respect and camaraderie, often expressed through playful challenges. She serves as a trusted advisor and caretaker for Master Makarov, viewing him as a father figure.

Beyond combat, Mirajane contributes significantly to Fairy Tail's daily operations as head waitress and enhances the guild's reputation through her public profile as a Sorcerer Magazine model. Her artistic talents include singing and guitar playing, performed during guild events. She is also skilled in cooking, a trait consistent since her youth.