Miledi Reisen, born into the noble Reisen family ruling the Reisen Gorge and its prison for the Grandort Empire, awakened gravity magic—one of the seven Ancient Magics from the Age of Gods—as a child. Forced to execute prisoners condemned as heretics by the Holy Church by hurling them into the gorge, this duty rendered her emotionless, expressionless, and nihilistic, viewing all defiance as meaningless since death was inevitable. Her father, Colt Reisen, treated her solely as a tool, devoid of affection while demanding strict adherence to her role.

A shift began when prisoner Davy Consman expressed pity for Miledi's inability to smile before leaping to his death, unsettling her. Later, Colt assigned Belle, a maid teaching etiquette who secretly belonged to the rebel Liberators under her true name, Belta Lievre. Initially cold, Miledi gradually warmed to Belta's kindness and playful nature, emulating her traits privately. This influence softened Miledi's execution methods, prompting Colt's investigation. Discovering Belta's identity, he arrested her. Despite Miledi's protests, Belta was executed via the gorge. Miledi attempted a rescue but arrived too late, witnessing monsters consuming Belta's body. In her final moments, Belta revealed the Holy Church's god Ehit was malevolent and urged Miledi to seek freedom and smile. Carrying Belta's body back, Miledi defied her father's order to discard it, rejecting her family's morals. Ordered executed herself, Miledi smiled for the first time, killed her family, freed imprisoned Liberators, and joined their cause.

As a Liberator leader dedicated to overthrowing Ehit, Miledi adopted a cheerful, mischievous facade, masking inner pain with humor and Belta's habit of nicknaming others. She prioritized freedom and resilience, smiling through hardship as Belta wished. Recruiting Oscar Orcus, a Synergist, forged a close bond marked by mutual teasing and respect, deepening romantically though neither initially confessed. They admitted their feelings before a critical mission but postponed pursuing them until after fighting the church. Miledi also formed strong ties with fellow Liberators Naiz Gruen Caliente, Meiru Melusine, and Laus Barn, the latter converting from enemy to ally after witnessing her resolve.

Miledi possessed extraordinary gravity magic, manipulating planetary energy, magnetic fields, and geothermal forces. Her spells included spatial manipulation ("Spatial Severance"), destructive attacks ("Star of Calamity"), and the ultimate "Heavencrush" tapping planetary energy. She mastered composite spells like "Whirling Sapphire" (gravity-compressed fire) and "Graviton Burst" (gravity-enhanced lightning), wielding a mana pool surpassing most humans and demons. Following the Liberators' defeat, she transferred her soul into a small mechanical golem with a smiley face and robes, enduring millennia alone to preserve their legacy. Overseeing the Great Reisen Labyrinth, she designed trials to test successors worthy of Ancient Magic, using intentionally frustrating messages that cemented her reputation as an "annoying" guardian.

In the present timeline, Miledi tested Hajime Nagumo's group within her labyrinth, granting gravity magic upon their success. She later aided their final battle against Ehit, expending nearly all magical power to create a black hole that destroyed the collapsing divine realm, saving Hajime and Yue. Her soul reverted to human form in her last moments, reuniting with deceased Liberators' spirits before dissipating. Posthumously honored as a hero, Miledi reincarnated in another world as Sheeni, a teen with a black bob-cut, brown eyes, and tanned skin. She eventually reunited with Oscar's reincarnation, regaining memories to begin a happier life.