Sent on a highly successful mission to conquer the Great Goering Nebula, Sincline returns to Daibazaal with vast treasures and grotesquely, the heads of the kings of the conquered nebula. But he is also quick to speak of rumors he has heard about the many defeats Galra has suffered at the hands of Golion. Showing for the first time the workings of his devious mind, Sincline prevails upon his father to spare the life of the disgraced Commander Sadak, who was about to be executed for his failures against the super-robot. In fact, he only spared Sadak so that he could be transformed by Sorceress Honerva into a giant deathblack beastman to use against Golion. This began a new phase in the team's battle to protect Altea from Galra. Whereas the battered planet had previously been so shattered, Sadak's mostly straightforward assaults had caused great damage and death just by striking at will, the Golion team's support had strengthened the world. Sincline's schemes invariably involved two or more different levels of assault, usually involving disabling or separating the team, since, once Golion was formed, the outcome was almost inevitable, though future Beastmen had weapons that made this less so.
In this first attack by Sincline, several recurring themes were established. The Prince Imperial challenged one of the team to single combat, claiming he wished to leave his father's ways behind and decide the conflict without massive loss of life. Despite knowing it was likely a trap (it was), team leader Kogane Akira agreed to the single combat and bested Sincline, only to become fully aware of his treachery when the combat was done. While Kogane lay unconscious, Sincline unleashed the giant Beastman Sadak had become, with the other four lions hard pressed to do anything against him until Kogane was revived by Princess Fala and rejoined the team in Black Lion. One other more ominous trend began when Sincline first saw Princess Fala, rapidly becoming obsessed with having her.
Successive attacks and traps showed the viciousness and depths Sincline was willing to descend to, but yielded no positive results for Galra. Even the constant aid and input of the evil sorceress Honerva only made their efforts a challenge for the Golion team, but never resulted in any real losses for them. His past triumphs began to be forgotten by his demanding father, and Sincline's own ambitions to see the throne were frustrated by the cunning and durability of his father (Episodes placed Daibazaal at over five hundred years old, and Sincline himself at a hundred and six). Despite numerous efforts and tricks, Sincline was never able to defeat Kogane in hand-to-hand combat, and his obsession with Fala proved a major distraction. Unknown to all but him, he was haunted by dreams in which a woman who resembled Fala was constantly kept away from him. Sincline even went to the extreme of capturing Princess Amue of Planet Heracles and holding her prisoner, since Amue was almost a twin for her distant cousin Fala. There is grim evidence she was abused in the worst way possible while a captive, though she held on until rescued by the unexpected emergence of Shirogane Ryou, brother of the fallen Go Lion pilot Shirogane Takashi. Sincline would develop a secondary rivalry with Ryou.
Sincline eventually found himself in the same position as the late Sadak, with his vile father ordering his arrest and execution for repeated failures, though he too needed a scapegoat for the empire's colossal failure to take this one world. Sincline however staged a palace coup, and placed his father as the lifeforce-bound pilot of MechaBeastman Galra. Golion might well have been destroyed by the giant in Daibazaal's image and likeness, but Sincline, seeking to secure his throne, sabotaged his own side, causing the Beastman and Daibazaal to be destroyed. To his shock, however, his own trap involving Human shields against Golion was in turn wrecked by the grieving Honerva.
It was then Honerva let loose with two shocking revelations. The first was that the late Daibazaal had been her own son. The second was that Sincline's mother had been Altean, like Fala, a thought which so disgusted the sorceress, she never once referred to Sincline as her grandson. Sincline also felt no connection to her, killing first her blue cat familiar, Jaga, and then Honerva herself, who warned as she died that Emperor Sincline would not reign long.
As Go Lion and resistance fighters led by Amue and Ryou surrounded the castle, a Sincline who was growing less sane by the moment killed all his troops and generals who suggested surrender. Pursued by the vengeful Shirogane Ryou into the wreckage of the castle, he instead captured his attacker and held his life against the behavior of the Go Lion team, demanding Go Lion be destroyed in exchange for Shirogane's life. When Sincline was distracted by the attack of the Space Mice, Ryou broke free, destroyed the beam that was melting Go Lion, and though slashed himself, managed to stab Sincline, pushing them both off the tower they stood on. Ryou did not survive the fall, but he twisted to make sure the tougher Sincline took the full impact. With Sincline's death, the war was at last over and the Galran Empire brought low by Go Lion. Presumably Sincline's body was destroyed by the heroic conquerors.