Audience Surrogate Newcomer

(may need a better name)

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. They are just an ordinary character, and often appear in Ensemble Cast works.

The purpose of these characters is to provide a mirror to the audience. At first, they give the audience someone to follow who they can identify with, as they enter an unfamiliar world. As such, they must appear at the beginning of a work, probably in a Welcome Episode. The character may decrease in importance as the series goes on, as the audience understands the world and doesn't need to ask questions. But they 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.

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 she 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, two factions of crew, but Harry doesn't yet belong to either group at first.