Milim Nava, a Dragonoid princess born millennia ago to Veldanava and Lucia in the Kingdom of Nasca, enjoyed early peace with her family and pet Gaia. Tragedy struck with her parents' deaths, leaving her largely ignored by her uncle, the king. Gaia became her sole companion until slain by assailants from the Super-Magic Kingdom of elves. This triggered Milim's rage, unleashing her inherited power. She destroyed the elves' capital, ascending to True Demon Lord via the resulting Harvest Festival. This event revived Gaia as a corrupted Chaos Dragon. Milim's rampage required Demon Lord Guy Crimson's intervention, culminating in a seven-day battle halted by Ramiris. This formed the Demon Lord council, with Guy, Milim, and Ramiris as founding members.

Initially, Milim acted as a protector, slaying monsters and arbitrating disputes. She assumed responsibility for the dragonewts, a race born from Gaia's essence, and the City of the Forgotten Dragon was built in her honor. Over time, disillusionment set in; she grew detached, seeking entertainment through the council and backing strong individuals for membership. Her interest in Rimuru Tempest sparked during the Orc Disaster, leading to their alliance. Milim presents as a youthful human girl, often mistaken for a child, with platinum-pink hair in twin tails and blue eyes. Her typical attire includes thigh-high socks, a chest-covering shirt, and black panties, sometimes with a cloak. A kid-friendly version features a longer shirt, light leggings, a black skirt, and shoes. Her true Dragonoid form manifests a crimson horn, wings, and jet-black battle armor.

Her personality blends fierce intelligence with childish exuberance. She is outspoken, disregards formalities, and seeks amusement, often neglecting duties. Though perceptive and impossible to deceive, she overlooks situational complexities for direct solutions. Insults about her appearance or age provoke violent reactions. Meeting Rimuru lessened her loneliness and jadedness; she embraced friendship and found joy in Gaia's revival. In "The Slime Diaries," these traits fuel comedic daily interactions within Tempest. She arrives unannounced, requiring Rimuru's supervision, and engages in activities like competitive potato digging or constructing snow sculptures. Her chaotic energy disrupts routines, yet she participates in seasonal events like harvest festivals and winter preparations.

Her powers include the Ultimate Skill [Wrathful King Satan] and [Dragon Eye], analyzing opponents' hidden energy levels. She employs abilities like Drago Buster, a homing beam attack, and effortlessly blocks strikes from powerful fighters. Despite her might, she respects Tempest's autonomy, acting only when permitted—such as defeating Charybdis after Rimuru's approval. In the spin-off, her strength is largely restrained for lighthearted scenarios.