Nebula appears to be the photo-negative of her sister: cruel, evil, and considered by men as strong as the Ravagers as the galaxy's biggest sadist. Despite turning against Thanos, she was still a criminal whose primary focus was what supported her in the moment (not unlike Loki). However, she has become more mellow and heroic in recent years, showing a selfless and heroic side as she willingly participates in the Infinity War.
Nebula is intelligent, but she appears not to be very good at creating strategies. For instance, she deserted Ronan when he was (currently) her only hope against Thanos, and she damaged her own ship attempting to shoot down Gamora.
Prior to the Infinity War, Nebula appeared to be very disloyal, as she ultimately betrays everyone and works for her own ends. Her exploits after the Battle of Xandar have been a journey of renewal for her as she begins to determine her own ideas of good and evil.
She is later seen in the raid on Knowhere, and chases Gamora's pod into the outer atmosphere where she destroys the pod. When Ronan decides to betray Thanos, Nebula asks him if he's going to kill his father, Ronan thinks she is going to oppose but instead she is allied with him promising if he kills Thanos she would help him to destroy thousands of planets.
Once the Guardians enter on Ronan's ship in Xandar, Nebula battles Gamora but escapes on a space pod, realizing that Ronan has lost the battle and has gone insane with power.
Nebula proceeds to live as a criminal until she is captured and placed in the custody of the Guardians of the Galaxy.
Nebula is tortured by Thanos as her father wants to force Gamora to relent and tell him where the Soul Stone can be found. Nebula later escapes and helps the Avengers and Guardians fight him, but they ultimately lose.
- Gal Gadot revealed that she was up for the role of Nebula, before it went to Karen Gillan.