Sendak was personally trained by Emperor Zarkon, and at some point received a cybernetic arm-and possibly eye-from Haggar's Druids. He was later entrusted with the Galaxy Garrison captives Shiro, Samuel Holt, and Matthew Holt, and managed to secure possession of the Red Lion of Voltron. After the Castle of Lions was discover on Arus, Sendak's ship was dispatched to it in order to attack and seize the remaining Voltron lions. However, the new paladins succeeded in forming Voltron and were able to bring down Sendak's ship, though he survived the crash along with his aide Haxus and several Galra Sentries. Following the defeat of the Myzax Robeast, he was contacted by Zarkon and given the opportunity to redeem himself, and soon sought to exploit the newfound friendship between the Defenders of the Universe and the Arusians.
Haxus' technical knowledge allowed him to alter a Galra Drone to mimic Rover, the Paladins' reprogrammed drone, which was then sent into the Castle of Lions and detonated to destroy the Balmera crystal that powered the castle. Sendak's forces then rigged a distraction at the Arusian village, making it look like it was under attack, which served to get Allura and Keith out of the castle. With Coran and Hunk leaving to find a new crystal with Pidge seeing them off and Lance injured in the explosion, only Shiro was left to see Sendak and his forces arriving with a corrupted crystal. Sendak and Shiro battled, and the commander recognized that Shiro had received a cybernetic from the Druids as well, but remained confident in his ability to win. However, the two ended up in a standoff, which was only broken when Haxus held the unconscious Lance at gunpoint, allowing Sendak to strike down Shiro, who was then restrained as Sendak's forces used their crystal to power up the castle.
Sendak's plans to take the castle and lions off planet were hindered by Pidge, who with Allura's guidance was able to sabotage the launch process despite Sendak sending Sentries after her. After Haxus fell to his death after fighting Pidge, Sendak contacted the Green Paladin and ordered her to surrender. She attempted to deceive him with a hologram but was found out, but the Space Mice had succeeded in letting Keith and Allura back into the castle, allowing Keith to attack. Sendak was then subdued by the Paladins-including a briefly revived Lance-and imprisoned aboard the ship in a cryo-pod. The Paladins later attempted to use Altean memory recording software to copy Sendak's memories, and were partially successful until Shiro jettisoned the pod, having suffered either a hallucination of Sendak or an illusion created by the corrupted A.I. of King Alfor.
Later in Kral Zera episode, Haggar used him to get control of the Galra Empire. But he did not succeed when Lotor and Paladins came to Kral Zera. And when Quartermaster Janka tried to escape, Sendak picked him up, dropped him to death and took his fleet.
After that he gathered supporters to seize areas of the universe, and they called themselves the Fire of Purification. When the Paladins they went three years ahead of time when they defeated Lotor, he conquered the Earth with his forces while Paladins were gone. He attacked there to get the Lions of Voltron. When Paladins returned to Earth, Haggar told him to destroy them. And he also tried to destroy the Earth. And for that, he built Zaiforge Cannons. But Paladins and IGF-Atlas' crew blocked his intention, and then Keith killed him.
Sendak was a cold and calculating Galra Commander, unfailingly loyal to Zarkon and determined to please his Emperor at any cost. He had no regard for innocents or perceived enemies of the Galra Empire, and no qualms about torturing or killing helpless prisoners. The illusion of Sendak attempted to convince Shiro that he should join the Galra, messing with his mind; this was likely based on Shiro's own experiences with Sendak.
Like all Galra Sendak was larger than most humans and presumably possessed superior strength. His entire left arm was a massive robotic implant, which could not only withstand from Shiro's less advanced version but also be extended to far greater distance on a stream of energy, allowing him to strike at enemies from further away.