Nebula

: "Thanos spent a long time trying to perfect me. And when he worked, he talked about his great plan. Even disassembled, I wanted to please him."
: ―Nebula to the Avengers[src]
Nebula is a former Luphomoid assassin, an adopted daughter of Thanos and adopted sister of Gamora. Together with Gamora, Nebula was assigned to Ronan the Accuser, during his quest to retrieve the Orb, although Gamora eventually betrayed them. Due to her hatred of Thanos, Nebula joined Ronan when he betrayed Thanos and tried to use the Power Stone by himself to destroy Xandar. During the Battle of Xandar, Nebula fought against the Guardians of the Galaxy but was forced to retreat from the battle. Later, Nebula was captured by the Sovereign and handed back to the Guardians, although she was able to escape and help the Ravager Taserface in a mutiny against Yondu Udonta. Nebula then attempted to kill Gamora, but they reconciled, so she helped the Guardians in their fight against Ego.
Nebula pursued her revenge mission against Thanos and even was able to infiltrate the Sanctuary II, but her attempt to kill him was failed. Thanos captured and tortured Nebula, using her as a bargaining tool to force Gamora to reveal the location of the Soul Stone. Nebula managed to escape and regrouped with the Guardians of the Galaxy and the Avengers on Titan, during their battle against Thanos, before she realized that he killed Gamora to get the Stone. The battle was eventually won by Thanos, who reached his goal and used the Infinity Stones to wipe out half of the universe's population, as Nebula was among the survivors. She and Tony Stark wandered in space onboard of the Benatar, awaiting their demise, before they were rescued by Captain Marvel.
As Thanos was eventually slain by Thor, Nebula and Rocket Raccoon joined the Avengers to uphold the order in the galaxy, while most of the Guardians of the Galaxy were gone. Five years later, Nebula participated in the Time Heist, as the Avengers attempted to get the Infinity Stones from alternate timelines and revert the Snap. Together with War Machine, Nebula retrieved the Power Stone, however, she got captured by the alternate Thanos, who found out about her universe and sought to bring his army there. Nebula was able to convince the alternate Gamora to turn against Thanos, and killed her alternate self to save Gamora and Hawkeye. She was among the participants of the Battle of Earth, and helped defeat Thanos and his armies.
After the Battle of Earth, Nebula and Rocket Raccoon parted ways with the Avengers and rejoined the Guardians of the Galaxy. Alongside Thor, they continued to help those in need across the galaxy, such as the Indigarrians during the Battle of Indigarr. While Thor parted ways with the team to fight Gorr the God Butcher, Nebula helped rebuild Knowhere into the Guardians' new headquarters. During this time, the Guardians marked their first Christmas celebration on Knowhere with Kevin Bacon.
Nebula later joined the Guardians on a new mission once Knowhere came under assault by Adam Warlock, who gravely wounded Rocket in a misguided attack on behalf of the High Evolutionary. As a kill switch inside Rocket hindered his recovery, the Guardians journeyed to the Orgoscope and Counter-Earth to locate an override code that would disable the switch. Nebula contacted the alternate Gamora and the Ravagers for reinforcements. After the High Evolutionary destroyed Counter-Earth, Nebula and the other Guardians devised the means to save Rocket, and then defeated the High Evolutionary before liberating his imprisoned subjects from Arête Laboratories. Following their victory, Nebula resigned from the Guardians to help raise the Star Children with Drax on Knowhere.