The last of his kind, Leo was conceived by his aged mother, Mothra, in the twilight years of her life. Focusing all of her remaining energy into birthing his mammoth egg, Mothra settled down to live out her days in peace. However, the evil Elia Belvera, in a plot to destroy what she felt were the "worthless humans," released the world destroyer, Desghidorah, from his ancient prison.

When the two Elias Moll and Lora learned of this they called in the aged Mothra to battle the armageddon beast. However, when she began to lose, the young Leo stirred, hatching prematurely in an attempt to aid his distressed mother. Born with increasable power, the new Mothra selflessly charged into battle with Desghidorah, confounding the beast by cloaking himself and using powers heretofore never seen by a larval Mothra. For a moment it seemed that the combined might of Leo and Mothra would prevail.

Yet, the battle almost ended in tragedy as Desghidorah, at the urging of Belvera, focused his dread attention solely on the larval Leo. It was only by the intervention of his mother that Leo escaped. However, there was a price to pay, as that final, selfless act of saving her only child drained the last strength from the old Mothra, and she plummeted into the ocean. Try as he might, Leo could not save his mother, and watched helplessly as his only family slowly sunk beneath the icy waves of the ocean. Now the last of his kind, Leo instinctively swam to an isolated island to begin his powerful metamorphosis.

After cocooning himself in the heart of an ancient forest, Leo emerged into his adult form as a swarm of multi-colored butterflies. As the butterflies coalesced into one massive insect, Leo took to the air and headed back towards Desghidorah, righteous furry burning in his wake. Arriving in a hail of energy beams, Leo relentlessly blasted his mother's murderer, throwing wave upon wave of searing beams and energy blasts at Desghidorah, who could only feebly attempt to defend himself against this, the most powerful Mothra of all time. Drawing upon an ancient legacy, Leo relentlessly assaulted Desghidorah, eventually renewing the seal that bound the world destroyer beneath the soil of the earth.

Yet, his work was not done with the end of the fight. Drawing upon the power of life that filled his very being, Leo restored to balance the blasted region deforested that was during the assault of Desghidorah. His work done for the time being, Leo retreated to his ancestral home.

A year later, Leo was called back into action when a toxic being, Dagarha, awoke from his ancient slumber. Fed by the poisons dumped into the oceans by humankind, Dagarha created thousands of Berums, acidic starfish that would consume all life in the sea. With righteous might on his side, Leo valiantly charged into battle with the toxic one, unleashing his ancient powers on the foul being. For a while it seemed like Leo had the upper hand, until Dagarha managed to force Leo underwater, where Dagarha was in his element, and Leo, lacking the ability to effectively move in the dense liquid, was caught at a severe disadvantage. Dagarha turned insult into injury when he created a cyclone of Berums, covering the noble Leo from head to abdomen in monstrous sea-stars. In pain and incapable of defending himself, Leo landed on the summit of an ancient temple that had arisen from under the sea, and waited for his foe to land the final blow.

Yet, just as all seemed lost, a small group of children, with the aid of Moll and Lora, managed to release the power of the ancient spirit Ghogo, empowering Leo once more and transforming him into the new, powerful, Rainbow Mothra.

After being beaten and encrusted with Berums by the toxic one, Dagahra, Leo landed on an ancient temple, awaiting his foe to end the fight. Yet, when a group of children, aided by Moll and Lora, released the power of an ancient water spirit, Ghogo, Leo was healed and transformed into the powerful Rainbow Mothra. Suffused with the water spirit's ancient magic, Leo re-entered the fray, relentlessly assaulting Dagarha. Yet, when the toxic one retreated below the waves, Leo followed suit and dove under the water, transforming himself into his new aqua form and finishing off Dagarha. With his foe defeated, Mothra once again flew off into the horizon, the world once again safe.

Leo's rest was interrupted again a year later, though, by the return of an ancient foe, the legendary King Ghidorah. Called into battle by his friends Moll and Lora, Leo valiantly fought the King of Terror, but to no avail, as even his extraordinary power was not enough to penetrate the wicked king's ancient defenses. Battered and weakened, Mothra was forced to retreat and come up with a new plan of attack.

Leo eventually hit on a new strategy to assault the King of Terror. Convincing Moll to use what was left of her powers, Mothra planned to send himself back through time to fight King Ghidorah when he was younger. Despite her protests, Moll eventually gave in, and, with the help of the triangle of wisdom, used her power to aid Leo in his journey back in time.

Arriving in the cretaceous period, Leo quickly located the younger King Ghidorah. Drawing upon all of his power, Mothra assaulted the young King of Terror relentlessly. Yet, without the power supplied by his contact with Moll and Lora, Leo was at a severe disadvantage against the brutal king. Just as it seemed that King Ghidorah was going to win, Lora managed to send her power to Leo, giving him enough strength to seize the immature King of Terror and drop him into an active volcano. However, the act took its toll on him, and Mothra was severely injured in the process. His quest completed, Leo crashed onto the hard cretaceous ground, his life slowly draining from him. As the light began to fade from his eyes, the larval forms of his ancestors surrounded him. Seeking to preserve their descendant, they encased him in a cocoon of silk, preserving his life and preparing him for his final, most powerful transformation.

When Dagahra fled beneath the waves during their battle, Leo, instead of waiting for his foe to reappear, used the power gained from Ghogo to transform himself into his powerful Aqua form. Diving into the ocean, Leo pursued the toxic beast, bombarding him with streams of brilliant energy. Then, in a surprising move, Mothra broke apart into his micro form, and actually dove inside the belly of the beast, lacerating his insides with pulse after pulse of surgically precise energy bolts causing the destruction of the monster's toxic Berums and flaying his internal organs. As the toxic one fell, Leo reemerged from the bowls of the beast, returning to his single form and emerging from the ocean before returning to his Rainbow form and dragging the carcass of Dagarha from the ocean.

Later, during the battle against King Ghidorah, Leo once again called upon his Aqua form, using its power as a springboard to transform himself into his time streaming form: Light Speed Mothra.

After Mothra Leo is defeated by King Ghidorah, the giant moth convinces Moll to help send him back 130 million years, so that he can defeat the King of Terror while the creature is in a much weaker form. The small Elias reluctantly agrees, and Leo flies off while Moll starts chanting to begin the process. Leo changes to Aqua Mothra, and then, with the help of Moll, changes to Light Speed Mothra and sends himself millions years back in time.

Though the plan worked, Mothra was stranded in the past, and Moll crystallized, due to spending the last of her vital energy to send him through time, Leo had to make sure that her sacrifice was not in vain as he prepared to fend off a younger Ghidorah.

Emerging from the cocoon his ancestors created for him back in the cretaceous, Leo broke free from the time capsule transformed him into his most deadly form: Armor Mothra. Spreading his razor edged wings, Leo took to the sky to fight the reborn King Ghidorah. Yet, this time the tables were turned, as his new armored shell reflected every assault King Ghidorah threw at him. Bombarding King Ghidorah with pulse after pulse of his energy bolts, Leo relentlessly battered the monstrous king, eventually severing one of the King of Terror's wings with his own razor sharp wing. Then, charging himself with the power of his ancestors, Leo flashed through the air, striking King Ghidorah and destroying him once and for all. The battle done, Leo released his armor, becoming Eternal Mothra.

After winning his fight with King Ghidorah, Leo transformed into this final form. Using the power of the Elias triangle, Mothra managed to revive his fallen ally, Moll. Then, after healing the small Elias, Leo flew off into the horizon, finally able to retire from battle permanently.