She strongly wishes to get her revenge on the Wizards of the Black Circle. In episode 9 of season 4, Nebula possesses Roxy through the White Circle and goes out to take down the fairy hunters herself with Roxy as her vessel.
The Winx Club trail the possessed Roxy to the warehouses on the docks where a battle royal between the Wizards and Roxy ensues. The Winx Club interrupts, much to Nebula's dismay. The wizards escape as the Winx Club are occupying Roxy wondering about her sudden personality change. Nebula threatens to make them pay as they gave the wizards an opportunity to escape by making a large sphere of energy surround her to presumably harm the Winx. The Winx Club surround her and make shields that reflect the energy of the sphere to Roxy and forces Nebula to leave Roxy, who faints out of having exhausted her powers in fighting the Wizards.
When the Major Fairy of the North, Aurora, took part in the Fairies' Revenge and started freezing the human world, Bloom challenged Nebula to a fight, the conditions being that if Bloom wins, the Wizards will have to be given a fair trial, while if nebula won the Winx will surrender the Wizards to the Earth Fairies for punishment and leave Earth. After a heated battle, Bloom won.
Her vendetta against the Wizards of the Black Circle leads her to overthrow Morgana as the Queen of the Fairies with Aisha as her second in command and to cross pass through Magix into the Omega Dimension to destroy the Wizards, after announcing to the Winx that she planned to lead the Fairies into taking the control of the world from the humans.
After the heated final battle with the Wizards who became defeated and frozen, she saw the error in her vengeance and wanted to stay in the Omega Dimension as a way to punish herself, telling Bloom that she had become alike to the Wizards. When Bloom told her that she was still Good, and that there still was light in her she returned with the Winx out of the Omega Dimension and was forgiven by Morgana, the Winx Club, and the Fairies of Earth.
Nebula then lead the Tir Nan Og Fairies in restoring magic on Earth so as to restore the balance of the forces of nature and allow new fairies to be born on Earth. Morgana decided to stay with her family and gave the Crown of the Earth Fairies to Nebula who she believed would do an excellent job protecting the Earth Fairies, making her the Queen of Tir Nan Og.
Ability-wise, she is extremely powerful as she gave Bloom a run for her money in a one-on-one fight in the episode "Bloom's Trial"of Season 4. This most likely makes her as powerful as Icy. Her attack spell Electric Storm seems to indicate some connection to electricity, being that thunder or lightning is a sign of judgment or war. Also she has shown to be very strong in using her telekinesis in battle and able to shoot very powerful purple, pink, and orange energy beams at her opponent. As the Major Fairy of War, Nebula seems to have the empathic ability to manipulate others to her cause to some extent making them wish to fight with her or against her, or it could be her personality. The size of her wings indicates that she is at least at the Enchantix level so that makes her a full-fledged Fairy. Nebula may have surpassed those fairy forms and attained a higher level fairy form, since when the Winx had to face the threat of Diana and Aurora, Miss Faragonda told them that Believix powers were ineffective and useless against Major Fairies and that they would need to use the Gifts of Destiny.
It is said in season 4 that she once was the Fairy of Peace, but due to her hatred of the Fairy Hunters, became the Major Fairy of War.
Story of Joining Sora's TeamEdit
Nebula had encountered Xion and Tsukune when they were searching for members-to-be, and she trusted them when the two helped her fight off Subspacers and Zelnemies. And when she was givin the Gem of Heart and was proud to be an honorary member and a main members. She had her first mission with Sora, Pluto, Aisha, and Stitch, they became face to face with Xigbar but becomes wounded by him, and when it was almost to late, Nebula started to fall in love with Sora when she was healed by him and saw his damaged-self after he kept protecting her.