Prince Sincline, crown prince of the Galra Empire and son of Emperor Daibazaal, possesses a hidden half-Altean heritage. His Altean mother, executed by Daibazaal during Sincline's childhood for pleading mercy to surrendered civilians, had her identity concealed from him. This concealed lineage becomes pivotal to his character.

He rules through pronounced sadism and mental instability, executing subordinates for minor failures or perceived slights and deriving pleasure from their deaths. His extreme ruthlessness includes imprisoning, starving, and implied rape of Princess Amue—a cousin of Princess Fala targeted due to her resemblance to his dead mother. This fixation extends to Princess Fala herself, driven by an Oedipus complex fueling his antagonism.

Sincline repeatedly disobeys Daibazaal's strategic commands, attempting to defeat the Golion team and capture Fala. After multiple failures, including a botched plan using Amue as bait, he exiles Daibazaal to battle Golion, seizing the throne and declaring himself emperor. Shortly after, Honerva—revealed as both his paternal grandmother and Daibazaal's mother—exposes his Altean blood. This revelation triggers a violent breakdown where he kills her.

In the final conflict, Sincline takes Ryou Shirogane hostage aboard his flagship while attempting to destroy Golion. He executes his own forces when they surrender, convinced by the Golion team. Blinded by Altean mice allied with the heroes during the confrontation, he is stabbed by Ryou; both fall to their deaths, ending his reign.

Beyond the main series, Sincline appears as a secondary antagonist in the non-canonical crossover game *Super Robot Wars W*. There, he manipulates events to exile Daibazaal, assumes the title of Emperor, and pilots a Mechablack Beastman Galra unit before being killed in combat against the player's faction.