The Underground Empire Tube was once a peaceful civilization beneath the Earth, but it was corrupted after Earth Emperor Zehba usurped the throne. Under his rule, Tube turns its power toward conquering the surface world.
Zehba's hatred extends to humans both above and below the Earth. His rule hides a monstrous secret: he is not truly human, but the child of the Lethal Dogler monster once defeated by Tube's original royal family.
Earth Emperor Zehba is the usurper who seized control of Tube and transformed the underground empire into a force of conquest. He presents himself as Tube's supreme ruler while hiding the truth of his monstrous origin.
His desire to dominate the surface world drives the war against the Maskmen, while his connection to the Lethal Dogler reveals the corruption at the heart of his reign.
Prince Igam is the true heir to Tube's throne and the twin sibling of Princess Ial. Raised as a man to preserve the royal line, Igam serves Tube as a loyal commander before eventually learning the truth behind Zehba's rule.
After confronting the consequences of Tube's actions, Igam abandons the empire and chooses a path of atonement.
Princess Ial is Igam's twin sister and a member of Tube's royal family. While living on the surface under the alias Mio, she meets Takeru and falls in love with him.
Her bond with Takeru becomes one of the central conflicts between the Maskmen and Tube, especially after Tube imprisons her in ice for loving a human.
Commander Baraba is a powerful swordsman of the Baluga Tribe and one of Tube's leading warriors. He relies on strength, combat skill, and direct confrontation in battle against the Maskmen.
Oyobur is a red-skinned, pointy-eared ninja of the Buyon Tribe. He serves Tube as a stealth-based operative and uses deception, speed, and unusual ninja techniques against the Maskmen.
Fuumin is a female ninja of the Fu Tribe who serves Prince Igam. She is a loyal and capable fighter who supports Tube's missions through stealth, combat, and battlefield tactics.
Anagmas is an elderly beast within Tube who possesses extensive knowledge of the underground empire and its secrets. He acts as an advisor figure among Tube's forces.
Thief Knight Kiros is a wandering warrior who appears later in the conflict. He is deeply in love with Princess Ial and becomes involved in the battle between Tube and the Maskmen because of his obsession with her.
Kiros is armed with a knife and sickle, and his special attack is the Crescent Screw.
Episodes 27-49
Okelampa is an insectoid support creature used by Tube. It fires an enlargement beam that allows Tube's Earth Empire Beasts to grow to giant size after being defeated.
Angler Soldiers are Tube's standard foot soldiers. They are black-skinned grunts with brown vine-like growths hanging from their heads and are deployed in large numbers against the Maskmen.
Earth Empire Beasts are underground monsters released from ancient frozen caverns. Many are created from Dogler beasts combined with parasitic organisms, giving Tube a wide variety of monstrous servants.
Angramon Fighters are Tube's fighter planes. They can unite into a centipede-like monster known as the Angramon Snake.
The Earth Imperial Castle is Tube's headquarters. During the finale, it rises to the Earth's surface as the conflict between Tube and the Maskmen reaches its final stage.