The heroes are not the only ones who can undergo change and rebirth. The forces of Evil can also recoup their power and grow with new strength. While the Rebels continue to struggle against Imperial tyranny, the Empire is constructing a new Death Star. A final confrontation must now take place. The forces of good, represented by Mon Mothma, leader of the Rebel Alliance, and those of evil, led by the Emperor, regroup to plan their strategies.
Luke discovers that Leia, who has guided and supported him throughout his journey, is his twin sister. In many ways she represents his positive "anima," the personification of the feminine aspects of his psyche. He also finds that he must confront Vader again. Yet when they make mind-to-mind contact through the Force, Vader appears uncertain rather than aggressive--a sign that he is beginning a transformation.
--from the Smithsonian introduction
The Admiral Ackbar costume is backed by a blow-up production painting of the Rebel briefing. I seem to remember reading somewhere that Lucas chose this costume as his Admiral due to its large, intelligent eyes.
The attributes of the Empire and Rebellion were consciously contrasted: while the Empire's Star Destroyers were bulky and based on right angles, this Mon Calamari cruiser is sleek, elegant, and covered in muted colors.
The costumes for the Imperial Royal Guards were some of the most intriguing of Return of the Jedi's original designs.
The Imperial Shuttle was used in Return of the Jedi, both as Vader's personal transport, and as the means of the Rebels' infiltration of the moon of Endor. The larger production model was one of the most complicated models produced for the Trilogy. Beside it is a smaller version, used for distance shots.