Robot faces are not human.
Although Nebula was always a strange character, she has grown up a lot since Guardians of the Galaxy (2014). She's now one her most popular characters, and her quest to befriend Gamora has been one of Endgame’s greatest moments.
The first thing that comes to mind is the way she looks in the comics, which have her resembling a giant Luphomoid with blue and purple skin. But she was a different creature in the past too, and she had a lot of different incarnations over the years.
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 her father (and she also had him as her uncle), but they weren't related at all. Their time together led to many problems with different enemies, even though there was no connection between them other than their shared mercenary status.
Her relationship with Thanos was complicated and ended up making her the biggest enemy. She was loyal to her father but resentful of his use and constant use against her.
Her resentment grew and Nebula was forced into combat with Gamora, her sister. When she beat her, Thanos would make her maim her and then replace a portion of her body with robotic enhancements. As she grew older, her resentment grew and she realized how much she disliked Thanos.
While she eventually made amends with her sister and was able make peace with Thanos, she still had difficulty with her adoption and the things Thanos did. This led her to become an unwilling ally of Guardians and turn against Thanos.
It's clear she was a major character in MCU. She's also a significant character in Endgame.
Her transformation into a hero is also surprisingly moving and unexpected. 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 is a blue-and-purple-skinned alien in the MCU. She was later made a Cyborg by 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
Guardians 2 Nebula tells of Thanos using her and Gamora to punish one another in 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.