Guardians of the Galaxy's Gamora and Nebula, two adopted daughters of the Mad Titan known simply as Thanos, the Avatar of Death. Gamora was born to a loving family and is the last of her kind, the Zen-Whoberis. Most of the species were exterminated by the Badoon, with the help of the Universal Church of Truth. Gamora's family was found and killed by Thanos. But he decided not to kill her. He saw something special in the child. Thanos adopts Gamora as his daughter, along with another, known as Nebula.
Nebula, a Luphomoid, was a brutal space pirate as well as close ally of Ronan the Accuser and Korath the Pursuer, both of whom serve for Thanos. As a child, she was able to seize control of Sanctuary II, a massive spaceship previously under the command of Thanos. The Mad Titan was impressed by Nebula's intelligence and brutality, but felt that she needed more strength, along with Gamora. He tortured the two of them, often near the brink of death, even going as far as to augment and convert Nebula into a cyborg, replacing her fleshy human arms with even stronger, completely mechanical arms.
Nebula despises Gamora with a burning, fiery passion. While Thanos doesn't mind Nebula, he loves Gamora, giving her all of the attention and gifts that Nebula wanted but never got at all. They were trained to become murderous assassins by Thanos, but he focused more on Gamora's skills than on Nebula's. He was clearly impressed by them, but not so much by Nebula's. He even went as far as to call Gamora not only his favorite daughter but his true daughter, enraging Nebula and causing her to hate Thanos. She lashed out at them one time and told Thanos that he's not her father and that he never will be. As punishment, Thanos locked her up in a chamber full of acidic water and dangerous creatures (stolen artifacts from Taneleer Tivan, the Collector) for an entire day so she will be a taught a true lesson in genuine, harsh, cruel pain. The water hurt her, and the creatures would always try to attack her. Gamora would often feel genuine sympathy for Nebula, but Nebula ignored her, feeling that Gamora was only faking her guilt.
Over the years, Nebula's opinions haven't changed. Ronan has been particularly frustrated by Thanos not taking Gamora's eventual betrayal seriously. Gamora has learned of Thanos making a deal with Ronan to destroy the planet Xandar in exchange for the Kree giving him an orb that has the ability to destroy planets. The orb also contains an Infinity Stone, one of the most powerful objects in existence. Thanos has had enough with Ronan. His politics bore him to death, his demeanor is that of a whiny and arrogant child, and he fails to take him seriously anymore. Fed up, Nebula and Ronan team up to destroy Xandar and use the orb against Thanos. Gamora goes to retrieve the orb on the planet Xandar, where she will meet up with people who will end up becoming her friends, but Nebula and Ronan plan on getting it first.