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.

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.

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.

Trivia

 * Gal Gadot revealed that she was up for the role of Nebula, before it went to Karen Gillan.