Post History
The most famous example of what you're describing is Sherlock Holmes, told of course from Dr. Watson's POV. Watson never becomes the protagonist of the story - the focus is always on Holmes, Watson...
Answer
#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/37441 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/37441 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
The most famous example of what you're describing is _Sherlock Holmes_, told of course from Dr. Watson's POV. Watson never becomes the protagonist of the story - the focus is always on Holmes, Watson serving merely as his "biographer". What Watson's perspective gives us is the incredulity at Homes's conjectures: where Holmes sees a solution to the mystery, Watson (and the reader) needs an explanation. Proceeding from the _Holmes_ example, your narrator can be a secondary character, so long as the MC is his main interest. Your narrator would then narrate only what is relevant to the story about the MC, his thoughts and feelings about the MC, rather than his thoughts and feelings about unrelated matters. Such POV is actually quite interesting, as it lets us see the MC from the "outside", as somebody close to him would see him, rather than from "inside". We'd never see his thoughts - only his actions, and what he is willing to tell his friend about his thoughts. It is helpful, I think, to think of the narrator as a biographer of sorts: he is telling the MC's story, not his own. But of course this role can be held by a family member, a close friend (like Watson), an official records-keeper for a (possibly imaginary) historical figure etc. So long as the narrator's goal, in-character, is to tell the MC's story, he remains a secondary character, never becoming the protagonist.