Nebula

"I’m a daughter of Thanos. Just like you."

- Nebula to Gamora

Nebula is a Marvel Comics character who is the adopted daughter of Thanos, and adoptive sister of Gamora.

Background
Like Gamora, Ebony Maw, Cull Obsidian, Corvus Glaive, and Proxima Midnight, Nebula was taken from her homeworld, an unnamed planet home to the Luphomoids, when Thanos massacred half of its population.

Personality
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.

Physical appearance
Nebula is a tall, slender, athletic, curvy, and beautiful blue-skinned humanoid cyborg woman with an oval head and black eyes. She is also bald (whether this is natural for Luphomoids, or the result of her cybernetic implants, is unclear). She has a robotic prosthetic arm and metallic implants around her left eye. She is clad in a black-and-purple jumpsuit that allows her complete freedom of movement while still keeping her completely modest. Her gloves, belt, and boots are all black and appear to be made of plastic. Her cybernetics, whilst given to her as a punishment, have proved to be an advantage: she can fix her body if she is injured using her cybernetics, as shown when Drax blasted her with the Hadron Enforcer, which was capable of destroying entire moons, and she plummeted an undisclosed amount of feet and rose to fight and nearly overpower Gamora moments after she landed.

Guardians of the Galaxy
Nebula is first seen aboard Ronan the Accuser's starship, asking to be put on the mission to retrieve the Orb, but shows some hostility when Gamora is chosen in her place. They tracked her to the Kyln, but as Gamora had already escaped, Nebula and Ronan massacred its residents together.

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.

Guardians of the Galaxy Vol. 2
Nebula proceeds to live as a criminal until she is captured and placed in the custody of the Guardians of the Galaxy. Thanks to Baby Groot, she eventually escapes and makes a deal with Taserface. Afterwards, she vows to kill Gamora and proceed to kill Thanos. She finds Gamora on Ego's Planet and attacks her, but after Gamora shows her mercy and saves her life, the two create an unlikely and temporary alliance against Ego.

Avengers: Infinity War
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. Nebula realizes what has happened when all the Guardians turn to dust except for Rocket.

Avengers: Endgame
Nebula will reappear in this film after the Decimation.

Trivia

 * Gal Gadot revealed that she was up for the role of Nebula, before it went to Karen Gillan.
 * Zoe Saldana (Gamora) has said the relationship between Gamora and Nebula is her favorite aspect of Guardians of the Galaxy,  and hits close to home for her, having sisters of her own.