What's the meaning behind John David Washington being known solely as The Protagonist in Tenet? Easily the maddest entry in Christopher Nolan's cinematic canon, Tenet's main character begins as a CIA agent, but gets made within the opening minutes. Luckily, his kidnapping is merely an audition for the clandestine Tenet organization - a test he passes with flying colors. For the rest of Tenet, The Protagonist embarks on an operation to save the world, preventing Sator from using the Algorithm and unwinding human history. The Protagonist goes back and forth through time, and during a final conversation with Neil, it becomes clear that John David Washington's story is only just beginning.
Tenet reveals plenty about its main character. Dedicated to the job, but not so much that he'll sacrifice his values. Quietly confident, like James Bond without the toxic masculinity, but quite happy to slap a man with a cheese grater. Intelligent enough (or watched Terminator, either one) to form a time-traveling organization in the future and send his own best friend back to close the causal loop. One thing the audience never learn about John David Washington's character, however, is his name. At no point is he ever referred to in specific terms, and Priya even uses the term "Protagonist" on screen.
Deliberately obscuring the main character's name is an odd choice at the best of times, but especially in a James Bond-flavored spy thriller, and there are a number of possible ways to interpret this bold decision, all of which could've played a role in Nolan's thinking. Firstly, Tenet is not a film that prioritizes character development highly, and the twisting narrative actually makes evolving the cast in a traditional sense all but impossible. As a result, The Protagonist acts as somewhat of a blank slate (though not necessarily uninteresting) hero for the audience to project themselves onto while watching. Washington arrives with no backstory, no deeper motivation, and experiences no personal journey in Tenet, other than saving the world. Naming him simply "The Protagonist" fits this general idea of a non-specific hero.
This concept works from an in-universe perspective too. In Tenet's world of espionage, names are more a hindrance than a help, and even "Neil" is probably a pseudonym (for Max?). Priya repeatedly asserts that in Tenet's line of business, ignorance is their advantage over the future, and the very nature of the mission remains strictly need-to-know. As such, having a name is an unnecessary distraction from the job at hand - especially when no one uses their real one anyway.
While this would explain why Nolan chose not to give John David Washington's hero a proper name, it doesn't account for why he's openly addressed as The (or, indeed a) Protagonist on-screen. Curiously, Nolan's Inception could be key to unraveling this mystery. Each of Cobb's main gang in the 2010 classic represent part of the film making process - Cobb is the director, Arthur the producer, Eames the actor, Fischer the audience, etc. Since Nolan is clearly interested in weaving meta themes into his films, it's only a small leap to call a lead character "The Protagonist."
A final possible explanation for The Protagonist's enigmatic moniker can be found in Tenet's unapologetic James Bond influence. With Tenet, Nolan is essentially modernizing the 007 formula, but avoiding the darker intensity of Daniel Craig's era. Picking a name for John David Washington's nu-Bond is inherently tricky; something with an upper class connotation falls too close to Ian Fleming's universe, but a simpler, modern name could be seen as taking cues from Jason Bourne. In fact, best to stay clear of "J. B." initials altogether. Taking a typically fresh view, calling Tenet's hero "The Protagonist" could be part of the film's overall ethos to be cool without trying to be cool. After all, it worked for Clint Eastwood in The Good, The Bad & The Ugly.