The Ultimate Blue is a Blue variant from the anime series Blue Gender. Appearing in the final two episodes of the series, this monster is the Blue species' apex form. It is a one-of-a-kind variant that serves as a guardian of the last Blue nest on Earth.
As the Blue Armageddon came to an end, the Blue underwent a mass extinction event, swarming to their largest nests and then rapidly fossilizing. Series protagonists Yuji Kaido and Marlene Angel, together with a band of volunteers from Second Earth, returned to Earth to investigate the Blue's migration across South America. They arrived to find that the Blue had died off, apparently having served the Earth's purpose by reducing the human population to a sustainable level. However, on disturbing the Blue graveyard, the humans inadvertently stirred a large Blue egg which hatched into the Ultimate Blue.
The Ultimate Blue was ferocious and powerful, displaying tremendous strength and speed. Its armoured chitin was tougher than any previously-encountered Blue and its tail and pincers could easily rend through Armoured Shrike mechs. Most disturbing of all, the Ultimate Blue had no core, meaning it had no easily exploitable weakness. Yuji and the team attempted to target the exposed joints and tendons, but the creature's frighteningly quick reflexes made this task exceedingly difficult. In the end, Yuji finished the battle by shoving his mech's weapon into the Ultimate Blue's maw and firing until he blasted the creature's brains out. Only he and Marlene survived.
After destroying the Ultimate Blue, Yuji attempted to use his own B-cells to establish a connection with the Earth itself, to try and appeal to its grand will. Should the Earth birth any more creatures like the Ultimate Blue, humanity would surely be destroyed, but Yuji was able to convince the Earth that mankind was no longer dangerous to it. Since the human population had been reduced to merely tens of thousands and the survivors had learned to survive without relying on dangerous technology, the Earth was convinced that humanity no longer posed a threat and so the Blue at last became extinct.
The Ultimate Blue is the apex of the Blue's evolution. It reached a height of 10 feet with a length of 25 feet, was covered with armoured chitin and moved on six powerful legs that allowed it to run astonishingly fast for a creature of its size and shape and even allowed it to jump great lengths and heights. It also possessed bladed forearms, a pair of large, scissor-like mandibles and a long tail that ended in a sharp pincer-stinger, capable of penetrating armoured vehicles. The Ultimate Blue was also the only Blue variant not to have a core, though there were gaps in its armour and patches of exposed musculature that were softer than the rest of its body.