In many genres, we expect the story to be told from a specific Point of View, with a certain character role that is expected to be The Protagonist. Some writers like to mix it up by choosing someone that does not have the central role in the story and tell the story from their perspective. This is the Supporting Protagonist: someone who would normally be a secondary character by conventions of the genre but is actually the main character in the story. When done correctly, this provides a Point of View other than what's typical. When done wrong, it can easily lead to the character becoming The Load or Pinball Protagonist devoid of any agency.
In many cases, this means choosing someone other than The Hero to be the protagonist. For instance, the story follows the Big Good as they watch The Hero on their adventure. In this case they are also the Supporting Leader. In another case the story could follow the Sidekick as they support the hero. The third case is Supporting Protagonist being The Hero, but not The Chosen One. It can also be that we follow this protagonist for much of the story, but the one who gets to resolve the in-story conflict is not him/her.
In a mystery, it means choosing someone other than the detective. In a Romance Novel, it means choosing someone other than the lead character. In Historical Fiction, it means choosing someone other than the important historical figure.
It's common in Japanese works with a supernatural touch (Light Novels, Fish out of Water scenarios, etc.) in order to have somebody to spout exposition to. In Real-Time Strategy games, this usually happens in conjunction with Non-Entity General.
This trope is often a good way to deconstruct the conventions of a given genre or character archetype, such as a "normal" person reacting to the many bizarre things that RPG characters tend to do, or a level-headed secretary constantly trying to propose saner alternatives to the Insane Admiral's shenannigans.
A sister trope to A Day in the Limelight, where just part of the story doesn't center around the expected protagonist, and First-Person Peripheral Narrator, in which the character is not the protagonist at all, but is the narrator of the story.
Compare Deuteragonist and Hero of Another Story. Contrast with Hero Protagonist and Decoy Protagonist (the character who appears at first to be The Protagonist but is not). See Secondary Character Title, when the character the work is named after isn't the protagonist.
