Emperor Geldon, also known as Emperor Geldon of the Gailar Empire, serves as the primary antagonist of the series. His background is that of a power-hungry figure from the planet Gailler. Originally serving as the commander of security for a Gaillerian expedition that crash-landed on Earth and remained in suspended animation, he seized his opportunity for power. After being awakened by human nuclear tests, Geldon murdered the legitimate sovereign of his people and usurped the throne, proclaiming himself emperor. He then established a hidden base of operations in the Arctic, from which he launched his campaign to conquer Earth.

In terms of personality, Geldon is depicted as a malevolent, treacherous, and tyrannical ruler. He is ruthless and ambitious, showing no regard for the lives of others in his quest for domination. His decision to murder his own leader to seize power demonstrates his complete lack of honor or loyalty. He rules his empire through fear and force, demanding absolute obedience from his subordinates. His motivations are driven by a desire to establish a new imperial domain on Earth, which he sees as a planet of limitless potential to be exploited for his own glory.

Geldon's role in the story is to serve as the source of conflict and the ultimate enemy that the protagonists Joe Kaisaka and Rita Yang must defeat. He commands the Gailar Empire's military forces in their ongoing assault on Earth, sending waves of attack robots and other weapons to terrorize humanity. To further his goals, he forced the pacifistic Doctor Yang, Rita's father and one of the most brilliant minds from Gailler, to build the ultimate weapon, the super robot Groizer X. When Doctor Yang secretly planned to use the robot to defend Earth and helped his daughter Rita escape with it, Geldon became obsessed with recapturing the machine and punishing the defector.

Key relationships define his role as a villain. His relationship with Doctor Yang is one of exploiter and victim, as he forces the scientist to work for him and later has him shot during his escape attempt. Rita Yang represents everything he wishes to crush: defiance, compassion, and the living proof of his failure to control the powerful Groizer X. He commands a hierarchy of military officers, including figures such as General Daggar, the treacherous Marshal Dogus, and the scientist Golem, who is responsible for creating the destructive Bomb Robots deployed from the fortress Gelmus. These subordinates carry out his orders for world domination, though some, like Dogus, are noted for their own cunning and ambitious schemes.

Regarding development, Geldon remains a consistently despotic figure throughout the series, serving as the unwavering threat that must be eliminated for peace to be achieved. He does not undergo a redemption arc or a change of heart. His narrative arc concludes with his eventual destruction, as he is killed in the final confrontation, finally receiving his comeuppance for the suffering he has caused. His death is a pivotal moment that allows for the bittersweet ending where peace can be established.

Emperor Geldon is not typically portrayed as a direct physical combatant but rather as the strategic mastermind and absolute authority behind the Gailar war machine. His notable abilities lie in his ruthless leadership, his willingness to commit any atrocity for power, and his capacity to command a vast, technologically advanced military force. He oversees the deployment of formidable weapons, including the air fortress Gelmus, from which his forces launch devastating attacks on Earth's cities. His ultimate power is the authority and military might he wields as the self-proclaimed emperor, which he uses without restraint to pursue his tyrannical goals.