The following contains spoilers for Black Panther: Wakanda Forever, now in theaters.
Namor, the Sub-Mariner, is one of Marvel Comics' oldest characters and has been everything from a hero to a villain. However, at his core, Namor is a king trying his best to protect his people. Whether this puts him at odds with old friends is rarely ever a concern for him. That said, the complexities of his character are detailed from his origin as being both a child of the sea and the surface. This makes him a mutant and the best person to handle the interactions between the two worlds.
While Namor's history is something that has defined his character and his personality, his name has also carried its own significance for himself and others who have heard it. However, all of this has been drastically changed in his debut in Black Panther: Wakanda Forever. Nevertheless, even though his character was altered, the film did nothing to change his reasoning or the importance of his name.
What Namor's Name Originally Meant in the Marvel Universe
When Namor appeared in Fantastic Four Annual #1 (by Stan Lee and Jack Kirby), his name meant "Avenging Son." While he may not have been known to many of the heroes prior to meeting the Fantastic Four, the meaning behind his name had already been established based on his actions against those who tried to harm his home or his people. Namor was an Avenger in the sense that he fought for many things that were taken from him and his people.
In Black Panther: Wakanda Forever, Namor's name took on a totally new meaning, it came from a Spanish Colonizer who saw Namor during his journey to bury his mother in her home on the surface. When he saw people in power oppressing the native inhabitants, he reacted by killing the colonizers. But one of the people who saw his actions called him "El Niño Sin Amor" (child without love). Embracing and shortening this name, he decided that those who oppose him would know him as Namor.
The MCU's Namor is Very Much Like His Comics Counterpart
Namor has always carried a major animosity for the surface world. But his motivations in the MCU are distinct from what he fought for in the comics. However, they still maintain his reasons for fighting. In the comics, Namor fought to protect his people from scientists who treated the ocean as a place for experimentation. Their actions led to the deaths of many of his people and, over time, led to the contamination of the oceans. As a result, he fought for those who couldn't and also fought to ensure the survival of his Kingdom.
In the MCU, Namor's hate for the surface came from the realization that it was filled with abusers and oppressors who thought they could rule over their own kind while polluting their oceans. Because of this, Namor hid his people from the surface but hoped to one day take the fight to them and eradicate all those who harmed those who couldn't protect themselves. In the end, while one was fueled by anger and the other fueled by a desire to make a better world, Namor is still a man who is tired of the surface failing its own kind, making him as complex as he is dangerous.