Robot faces are not human.
Nebula has always been a bit of an oddball, but she's also grown a lot since 2014's Guardians of the Galaxy. She's now one her most popular characters, and her quest to befriend Gamora has been one of Endgame’s greatest moments.
First, the way she looks in comics makes her look like a giant Luphomoid with purple and blue skin. She was, however, a very different creature in the olden days and has had many different incarnations.
She was also a pirate by trade, so she enjoyed many adventures throughout the universe. In some of her more cosmic Marvel stories she even fought the Skrulls.
Thanos was her father (and she also had him as her uncle), but they weren't related at all. They were in a lot of trouble together despite the fact that they were both soldiers.
Her relationship with Thanos was complicated and ended up making her the biggest enemy. Although she was loyal to her adoptive father, she felt resentful at his use of her as an instrument against others.
Nebula's resentment grew. She was forced by Gamora to compete with her in combat. Thanos would also maim her and implant robotic enhancements into a part her body. 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. This led her to become an unwilling ally of Guardians and turn against Thanos.
It's pretty obvious that she was a very powerful character in the MCU, and she was also one of the first to join the Guardians in the film. She's also an important character in Endgame.
Surprisingly, and even unexpectedly, her transformation to a hero is quite remarkable. It's also quite possible that she will be the final death of Thanos, as she claims to do at the end of the film.
How she became an iCyborg
Nebula, an alien with purple and blue skin was the original Nebula in the MCU. However, Thanos' evil mind made her a cyborg. Her condition led to her being subjected a variety of surgeries, bionic and other bizarre enhancements.
How she changed throughout the MCU
Guardians 2 tells us Nebula that Thanos used Gamora, her sister, and her to punish their fighting match winner. He would remove a portion of their bodies and replace it with some kind of robotic enhancement. This made them look awful.