He only recently made his debut in the MCU, but Namor has been around in the comics long enough to be considered a brother by none other than Captain America. The two heroes fought in World War II together as members of the Invaders, making them true brothers in arms. It's unfortunate that this relationship will never get to play out on-screen.
Namor is among the very first super-powered characters in Marvel Comics. Created by Bill Everett, Namor the Sub-Mariner made his debut in Marvel Comics #1 in 1939, predating Captain America’s debut by roughly a year and a half. Although much more of an antihero in his initial stories, Namor found himself joining the fight against the Nazis when America entered WWII, often working in tandem with Cap, Bucky and the Human Torch. Namor remained one of Marvel’s most popular characters throughout the 1940s, but fell out of favor following the end of WWII. Stan Lee and Jack Kirby brought Namor back in the 1960s in Fantastic Four, returning the character to his antagonistic ways by portraying him as an antihero and, sometimes, an out-and-out villain. Yet in spite of all this, Namor retained the respect and admiration of Captain America
The limits of Cap and Namor’s relationship are tested in 2012's New Avengers Vol. 2 #29, by Brian Michael Bendis and Mike Deodato, Jr. Taking place during the Avengers vs. X-Men crossover, the story finds Cap assembling the ultra-secret Illuminati to see if there is any way they can put an end to the conflict between the two teams. All the members gather together except for Namor, who shows up last. Cap pleads for him and the other Marvel heroes who make up the Illuminati to put an end to the conflict, but Namor says that the world is changing for the better and Steve is doing little more than protecting the status quo. Captain America then recalls a moment from the war during the Battle of Anzio when Namor saved him from one of the Red Skull’s war machines, proclaiming that Steve is his “brother.”
Namor and Captain America: Brothers in Arms
Even though their differing ideologies and allegiances often put them at odds, the fact that the two characters still respect each other enough to call themselves brothers speaks volumes of their relationship. The flashback in New Avengers #29 of Namor proclaiming his oldest ally Captain America as his brother is significant all on its own. Namor is often depicted as one of the most arrogant characters in all of Marvel Comics, just as likely to show any sort of vulnerability as he is to sacrifice one of his limbs. The Sub-Mariner isn’t going to name just anyone a “brother,” let alone tell them to their face. Even though they find themselves on opposite sides from time to time, the respect between the two heroes runs deep.
Namor did not make his first appearance in the MCU until 2022's Black Panther: Wakanda Forever, years after Steve Rogers gave up the shield of Captain America in Avengers: Endgame. Therefore, it seems unlikely that Captain America and Namor will ever cross paths in the MCU, but their relationship in the comics remains for all time.