Milim Nava, daughter of the Star King Dragon Veldanava and Lucia, sister to the first emperor of the Eastern Empire, emerged millennia ago as the world's sole Dragonoid, inheriting immense power. She resided peacefully in the Kingdom of Nasca with her parents and companion Gaia, a dragon gifted by Veldanava. This peace shattered tragically with her parents' deaths, leaving Gaia as her sole connection. Raised unaware of her lineage by servants while her uncle, the king, refused to acknowledge her, her bond with Gaia deepened.

Agents from the Super-Magic Kingdom of elves infiltrated Nasca seeking to exploit Milim's power, killing Gaia while defending her. Consumed by grief and rage, Milim unleashed her latent power, annihilating the elven capital and its inhabitants. This rampage triggered her evolution into a True Demon Lord during the Harvest Festival, absorbing countless souls. The process resurrected Gaia, but corrupted by Milim's fury into a mindless Chaos Dragon that continued the destruction under her command. The devastation threatened the world, prompting Demon Lord Guy Crimson to intervene. He fought Milim to a standstill for seven days and nights until Elemental Queen Ramiris pacified her, sacrificing her own purity. Regaining her senses, a sorrowful Milim sealed the corrupted Gaia away. She then co-founded the Demon Lord council with Guy and Ramiris.

Seeking redemption, Milim protected towns, arbitrated disputes, and aided those in need. Discovering the dragonewts, a race born from Gaia's essence, she took them under her protection. They revered her, building the City of the Forgotten Dragon in her honor where she resided, largely delegating governance to priests like Hermes and Middray. Over centuries, witnessing persistent conflict and betrayal eroded her faith in the world. She grew increasingly jaded and reclusive, neglecting duties to seek entertainment, particularly by fostering potential rivals among Demon Lords hoping for a challenging fight.

Milim typically maintains a Concealed Form resembling a youthful human girl, often mistaken for ages 12-15, with long, glossy platinum pink hair usually in twin tails and bright blue eyes. Her standard outfit is bold and revealing: thigh-high socks, black panties, and a long-sleeved shirt covering only her chest, sometimes with a large cloak. Variations exist, such as a toned-down version in children's media featuring a longer shirt, skirt, leggings, and shoes. Her True Form reveals her Dragonoid nature with crimson horns, large wings, and jet-black battle armor. Releasing her power limiter activates Hell Mode or Stampede Form, marked by darker skin, fiery red and black organic armor, claw-like gauntlets, greaves, dragon wings, a tail, and glowing blue eyes, signifying immense destructive potential.

Outwardly energetic, childish, and mischievous, Milim speaks bluntly, disregards formalities, acts impulsively, and seeks constant stimulation to avoid boredom, often shirking responsibilities. She possesses a notorious sweet tooth, especially for honey, and strongly dislikes vegetables, particularly raw green peppers and carrots, stemming from constant offerings as worship food. This facade masks a fiercely intelligent, observant, and strategic mind. She is difficult to deceive, successfully tricking powerful figures like Clayman and other Demon Lords by feigning mind control. Despite her intellect, impulsiveness and self-centered focus often lead her to overlook complex situations for direct, sometimes destructive, solutions. She reacts violently to gossip or insults. Underneath, she avoids unnecessary harm and deeply values genuine friendship. Her lonely and violent tendencies lessened significantly after forming bonds, particularly with Rimuru Tempest. Gaia's eventual revival filled her with profound happiness, further mellowing her.

Milim wields vast, world-threatening power, earning the titles "Destroyer" and "Destruction." Key abilities include Unique Skills: Dragon Eye for truth perception and analytical sight, and Dragon Ear for exceptional auditory perception. Her Ultimate Skill is Wrath King Sataniel, providing limitless magic energy. Destructive techniques include Dragon Star Burst, capable of obliterating cities, and Dragon Star Nova, which can annihilate entire capitals. She employs Multi-layer Barriers for near-absolute defense against attacks and mental interference. Her power limiter can be released for Hell Mode's enhanced combat capability. Full loss of control triggers Stampede, transforming her into a global threat. She wears Dragon Knuckles, gloves crafted by Rimuru Tempest, to better regulate her strength.

Milim first encountered Rimuru Tempest through Clayman's scheme involving the Orc Lord incident to create an artificial Demon Lord. Intrigued by Rimuru's strength, she visited Tempest to recruit him. An initial conflict arose when Rimuru's subordinates attacked her, leading to a near-devastating fight. Rimuru pacified her with honey, resulting in a truce and the start of a deep friendship. She considers Rimuru her "mabudachi" and frequently visits Tempest for food, games, and the labyrinth, where she serves as a boss. She values her friendship with Veldora Tempest, her uncle, though their combined chaotic energy often requires supervision. Frey serves as her supervisor and friend after Clayman's defeat, while Carrion (Karion) became her aide following his loss to her. Her relationships with ancient Demon Lords Guy Crimson and Ramiris are complex, marked by past conflict and the council's founding. She maintains a distant but protective connection to the dragonewts in the City of the Forgotten Dragon.

During Clayman's plot to undermine Tempest and become a true Demon Lord, Milim pretended to be under his control, gathering intelligence while secretly aiding Rimuru. She revealed her deception at Walpurgis, turning against Clayman and crucially contributing to his downfall. This led to Frey and Karion formally entering her service, expanding her territory. She participated in building and managing Tempest's labyrinth. Rimuru later facilitated Gaia's revival by providing Milim with an "egg" containing its soul, significantly aiding her emotional healing and happiness. Despite efforts to control her strength, visits to Tempest or other realms often involve accidental property damage. She continues to evolve, showing greater emotional stability, loyalty to friends, and involvement in their endeavors while retaining her core playful and powerful nature.