In the anime, she lost one of her arms as a child during an Ohmu attack, and is driven by a desire for revenge against the giant insects. She intends to wipe the Sea of Corruption off the face of the earth, by any means necessary. While she is not presented as completely evil, she is definitely an antagonist. Kushana's goal in life is to destroy the Fukais all the Omhus in exsistence. Her motive behind her determination is the fact that when she was a child, she was attacked by an Omhu, and as a result, her arm and two legs were amputated. She had to grow up with a metal shell as an arm and has been very angry ever since. Although at times she appears to be mean and cruel, she is actually very afraid. She puts up a strong front in order to hide her fears and actually has a very caring heart. As the story unfolded, she changed into a gentle and more caring person. When Mito returned to the Valley, she ordered her troops to stop firing at him because she was worried about Nausicaä's safety. So even though she led the invasion of the Valley, Kushana is not a hateful character. Her plight generates sympathy, and her goals and ideals are understandable. She could not have predicted the results of her actions and did not do anything out of cruelty or evilness.
In the manga, she has a much more complicated role, more of an antihero/ally than an antagonist. She is mostly driven by a desire for revenge against her father and three older half-brothers, whom she hates and blames for the fate of her mother. She commands the Torumekian Third Army, whom she trained personally; she is devoted to her men, and they are unswervingly loyal to her. Despite her ferocity in battle, she has a nobility of character that earns her the respect of both Nausicaa and Master Yupa. While she admires Nausicaa's pacifist ideals, she sees herself as inescapably destined to walk the path of bloodshed. She cares deeply for the men under her command, but is portrayed as brutal and harsh to her enemies. Hideaki Anno, who worked on the film of Nausicaa, had planned to do a manga side story centered around Kushana's military exploits, but Miyazaki refused him permission, believing that Anno would only use her to "play war games".Susan J. Napier and Patrick Drazen note a parallel between the character of Kushana, the rival warrior princess, and that of Nausicaä - Napier describes Kushana as Nausicaä's "shadow", noting that Kushana is not shown with any "alleviating, "feminine" virtues" as Nausicaä is, but that they share the same tactical brilliance. Drazen describes this as a "feminine duality". Miyazaki has described the two characters as being "two sides of the same coin", but Kushana has "deep, physical wounds". Kushana is one of my favorite characters in the entire series, and rightfully so. Just like Nausicaa, she was born into a man's world. She had to survive by, in a sense, becoming a man; she wore armor, commanded fleets, etc. She had to make tough decisions, and had to be very cautious of her surroundings. Because she knew, behind everything, her brothers were out to kill her and take over the thrown.
While seemingly cold-hearted at first, the reader's approach to the character becomes more emotional as we delve into the plot. We find out that the new emperor after Kushana's father poisoned Kushana's mother, making her insane. People took Kushana away, and left her mother; alone, with only a rag doll. In her insanity, Kushana's mother thought that this rag doll was her daughter. This part of the story once again reflects upon greek mythology, where many of the gods had killed their children in temporary insanity.
Kushana was forced under marriage by the Holy Dorok Emporer after being captured, though she kills him by, well.. slicing off his head. Actually, the slicing off of his head really didn't kill him.