Only a robot's face
While Nebula has been an oddball her whole life, she's grown a lot in the years since Guardians of the Galaxy 2014. Now, she's one of the most likable characters in the MCU, and her journey to befriend Gamora is one of the best things about Endgame.
First, the way she looks in comics makes her look like a giant Luphomoid with purple and blue skin. However, she was a completely different creature in the past and had many different incarnations throughout her life.
She was also a pirate, so she had many adventures in the universe. She even fought the Skrulls in some of the more cosmic Marvel stories.
Thanos was Thanos' father in the MCU. She also had him as her uncle. But she wasn’t really related to him. During their time together, they got into all kinds of trouble with various enemies -- even though they had no real connection other than the fact that they were both mercenaries.
Her relationship to Thanos was complex and eventually became her biggest enemy. She was loyal to her father but resentful of his use and constant use against her.
As her resentment grew, Nebula was forced to compete with her sister Gamora in combat, and when she beat her, Thanos would maim her and replace a part of her body with robotic enhancements. As she grew older, the resentment got worse and she began to dislike Thanos.
She made amends with her sibling, but still had difficulties with Thanos and her adoption. She became a reluctant ally for the Guardians after turning against Thanos.
It is obvious that she was an extremely powerful character in MCU. She was also the first Guardian to join in the film. She's also a major character in Endgame, and she's one of the more emotional characters.
The transformation she makes into a superhero is both surprising and moving. It's possible that she will become Thanos’ last victim, as she claims to at the end.
How she became Cyborg
In the MCU, Nebula was originally an alien with blue and purple skin, but she was made a cyborg thanks to Thanos' evil brain. Due to this, she was subjected to all sorts surgery, bionic improvements, and a host of other strange stuff.
How she changed during MCU
In Guardians 2, Nebula tells us that Thanos used her and her sister Gamora to punish the winner of their fighting matches. He would take off a section of their bodies and place it in some form of robotic enhancement. They both looked pretty horrible.