The dragon is a wind dragon.
Milim brought an adult wind dragon dungeon that she tamed, where it guards the 97th floor of the Labyrinth. It was allowed to make a nest and it feeds off of Veldora's aura for food.
From fighting in the labyrinth and absorbing Veldora's aura the dragon evolved into a dragon king class.
The dragon was granted a name by Ramiris and Rimuru gave them 5000 souls, which allowed it to evolve into a True Dragon King. Upon its evolution, it obtained a humanoid form, which is different from Milim's draganoid form in that Milim is a Spirit Form species, while the Dragons are a corporeal species that have changed forms (they are bound to their bodies). Though it had equal stores of energy to the Ten Lords of the Labyrinth, had lower fighting prowess, and thus decided to train in its new body.
Dragon - 20m dragon (assumed).
Dragon King - 20m dragon (assumed).
True Dragon King - Beautiful Humanoid form.
Wind Elemental Control: Presumably can use lightning.
|Ch||Evolution Stage||Trigger for Evolution||Rank||EP||New Skills/Abilities|
|Adult Dragon||Collected by Milim||A||Battle||Physical attacks with fangs and talons.
Elemental attacks - Wind (lightning)
|146||Veldora Aura and Fighting||A+|
|164||Dragon King||Name by Ramiris
5000 souls by Rimuru