Dester is the main antagonist of Nobita and the Robot Kingdom.
When King Atom died due to an accident, Dester manipulated Queen Jeanne into believing that the cause of the king's death was because all robots have emotions, causing the queen to order a law to remove all emotions in robots. However, he suspects Jeanne still thinks Maria as her mother-figure because she hasn't ordered the soldiers to remove her emotions even when she was imprisoned. Later, after capturing Doraemon, he forced him into having a fight with Kongfighter to know about the power of the earth robot.
When the battle resulted in Doraemon's victory and Doraemon escapes with his friends of his hands, he and Jeanne orders to follow him, but near a cliff, he causes a robot malfunction to throw the Queen and usurp the Kingdom and manages to convince the entire council that she is dead and making him the king. In the end, his mask is destroyed and it's revealed that he is the evil twin of Chapek, who uses science in the wrong way and is imprisoned.
Comparison between Manga and Movie
In the Manga, Dester's large body revealed to be a mini-mecha piloted by Desta himself. Also, when Desta escaped from the rocket, he had a short fight with the gang only to be subdued with an Air Cannon, destroying his mecha and knocking him out in the process.
In the movie, he is just wearing armor and was weakened by Doraemon who accidentally rammed him with his head earlier, and when he woke up and came out from the portal, he's still in pain that he passed out again before the gang had the chance to subdue him.