Audience Surrogate Newcomer

In short, Supporting Protagonist meets Audience Surrogate.

There's some world that is already happening, and this character gets thrown in, in medias res, which is an awful lot like Real Life can be. The important thing is that they are not the Protagonist, and are definitely not The Chosen One -- like 99.9% of the audience. They are just an ordinary character, and often appear in Ensemble Cast works.

The purpose of these characters is to provide a viewpoint to the audience. At first, they give the audience someone to follow who they can identify with, as they enter an unfamiliar world. The newcomer reasonably ask questions so the audience can get its exposition, too. As such, they must appear at the beginning of a work -- often in the very first scene, and probably in a Welcome Episode. The character may decrease in importance as the series goes on, once the audience becomes acclimated to the setting. But they still provide a subtle anchor that reminds the audience of their own journey along with that character.

They are often, but not always, a Naive Newcomer -- the naivete is optional here. Fish Out of Water tropes are common. They may be The Watson. Closely related to Deuteragonist. Contrast Chosen One and Coming of Age Story, where the newcomer grows into the lead role. For shows that start off like this, then veer off towards two lead characters fulfilling this role to each other's milleu, see instead Wunza Plot.

Anime and Manga

 * Kagome in Inuyasha is perhaps the Deuteragonist, but as an Ordinary High School Student in the mythical Jidai Geki, she provides someone for the audience to relate to.
 * Togusa in Ghost in the Shell: Stand Alone Complex is the newest member of Public Security Section 9, and an unaugmented human to boot.
 * Averted in Super GALS!, where it looks like Aya is going to be in the shadow of gregarious Ran the entire time. And indeed, the story revolves around Ran -- but Ran comes out at the end exactly the same person as she was in the beginning.  It turns out Aya was the Protagonist all along, as she comes of age.

Film

 * Buzz Lightyear plays this role in the first Toy Story film. He's not even aware he's a toy at first, so he learns the world of the bedroom toybox at the same time as the audience.

Live-Action TV

 * Noah Wylie's Dr. John Carter in ER is a first-year resident in the hospital, getting thrown in the deep end.
 * Sheriff Jack Carter in Eureka has to keep the peace in a town full of science experiments he doesn't understand, but the show quickly settles into an Ensemble Cast
 * Harry Kim in Star Trek: Voyager transfers to the USS Voyager right before she's sucked in to the Delta Quadrant, and for the first year of the series tries to work his way into the existing crew relationships. Although with the Maquis present, there are two factions of crew, but Harry doesn't yet belong to either group at first.
 * Joel Fleischman in Northern Exposure is the new doctor paying off his student loans through practicing in the Quirky Town of Cicely, Alaska.