It’s going to become an extremely long and easily-added-to list if we say that DGHDA counts. After all Dirk appears in or is mentioned in pretty much every scene in that book from the first to the last. He interacts with every single character as far as i recall and his character (not just his existence) is solely responsible for moving the plot along and his character evolves as much as any other character. We know approximately as much about Dirk’s actions and intentions within the story as we know about the other characters.
That differs form the other examples where the title character doesn’t even appear in numerous key scenes or is even dead or missing when the scenes occur. The deliberate actions of those characters have little effect on the course of much of the story. Andof ocurse we don’t actually get to know as much about most of those characters, even Caesar, as we do about the real hero.
If we include DGHDA then we need to include almost every story that isn’t essentially a one-person ‘biography’ like “Forrest Gump”. So throw in all Sherlock Holmes (they primarily focus on Waston and the ‘victims’), throw in Return of the King (we never know Aragorn’s thoughts either and he is certainly secondary to Frodo and even Gandalf). Add even throw in “Return of the Jedi”.
While Dirk and Aragorn and Luke may not be “the” main protagonists of their stories they are certainly major characters in the unfolding story. There’s going to be some argument about when a characeter ceases to be major of course, but I think that any character that appears in or is directly instrumental in setting up (or failing to prevent) every major scene in a story is major. And any charcter that gets apporoximately equal “screen time” as the other also should count IMO.
In that respect we could also add The Lion, The Witch and the Wardrobe where the children are obviously the “heroes” and the lion and witch don’t have much showing . Also Moby Dick since the whale really serves only as foil/goal, not a character per se.