Just a robot face
Although Nebula was always a strange character, she has grown up a lot since Guardians of the Galaxy (2014). She is now one of Endgame’s most beloved characters. Gamora was her best friend and she has become one of their most beloved characters.
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 an entirely different creature in past times, and she has been through many different versions over the years.
She was also a pirate by trade, so she enjoyed many adventures throughout the universe. In some of the more cosmic Marvel stories, she even fought the Skrulls.
Thanos was Thanos her father in MCU (and she had her as her uncle too), but she wasn’t related to him. They had a lot of problems together, even though they didn't have any connection beyond the fact they were both mercenaries.
It was a difficult relationship for her with Thanos, which ended up being her greatest enemy. Despite being loyal to her adopted father she became resentful towards him and his constant use her as a weapon in the war against others.
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. This resentment only intensified as she grew older and started to realize how much she didn't like Thanos.
Although she made amends with the sister she loved, she was still bitter about Thanos's adoption of her and other mistreatments. That led her to turn against Thanos and become a reluctant ally of the Guardians.
It is clear that she was a strong character in MCU. In fact, she was one of the Guardians' first members. 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 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 a cyborg
Nebula, a blue-eyed alien with purple skin, was initially a member of the MCU. Thanos changed her mind and made her a cyborg. Her condition led to her being subjected a variety of surgeries, bionic and other bizarre enhancements.
How she changed during 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.