With a Friend and a Stranger

When introducing characters, you want to keep the overall number of relationships and names to remember to a minimum. This is why many media start off with only a handful of primary characters, often three. There are many common "stock" variations on the genders and relationships of these few characters.

One of them is the friend and stranger dynamic. The protagonist will have a best friend, often a childhood friend. This dynamic is easy to show in the limited time you have to hook your audience, and easy to add depth to at a later time. The third character will be a stranger to the protagonist and his friend. His or her appearance will often serve to drive the plot, and his unfamiliarity with the characters and location allows for Exposition.

On occasion, it will be the protagonist who is the stranger, also acting as a Audience Surrogate to understanding the setting as well as fulfilling the stranger role in this trope.

The stranger can also put a strain on the friends, since he or she changes their long-standing dynamic, especially if they become involved in a Love Triangle.

Tropes that may play out in these scenarios when both the friend and stranger are female include Childhood Friend Romance, First Girl Wins, and The Dulcinea Effect.

In Eastern RPGs, this dynamic almost always appears as a (male) hero, his childhood friend, and a girl from nowhere (almost always female and a Mysterious Waif).

This adheres to the 3 dynamics in Cast Calculus. They may form a Power Trio.

See also Two Guys and a Girl, Tomboy and Girly Girl. Compare to Three Amigos. If the protagonist is the stranger, he's probably also a Rookie Red Ranger.

Anime and Manga

 * Star Driver: Takuto, the stranger with Wako and Sugata (childhood friends).
 * Outlaw Star: Gene, Jim, and Melfina.
 * Tengen Toppa Gurren Lagann has Simon and Kamina meet Yoko in the first episode.
 * The Advanced Generation and Diamond & Pearl arcs of the Pokémon anime can fit, with Ash being the main character, Brock being the friend, and May/Dawn the stranger.
 * Lupin III: Lupin and Jigen are long-time partners in crime. Fujiko is the mysterious stranger.
 * In the Mai-Otome anime, Arika is the stranger as a new student at Garderobe and Nina and Erstin are the friends. In the manga, "Manshiro" is the stranger, and Arika, Nina and Erstin are the friends.

Film - Animated

 * Monster House follows this dynamic to a T, with the protagonist DJ, his best friend Chowder, and Jenny, a girl they meet that day and who gets too close to the titular house.
 * In Tangled, Rapunzel (main character) sets out on her adventure with the chameleon Pascal (friend) and Flynn Rider (stranger).

Film - Live Action

 * In National Treasure, Ben tries to figure out the clues with his buddy Riley, and when they steal the Declaration of Independence, Abagail, a woman who worked with the document, gets dragged along for the ride.

Live Action TV

 * Pushing Daisies has Ned (main character), Chuck (childhood crush/neighbor), and Emerson (PI).
 * Lizzie McGuire: Lizzie (the main character), Gordo (the Childhood Friend Love Interest of Lizzie) and Miranda (the stranger).
 * Buffy the Vampire Slayer starts out like this, with new girl Buffy becoming friends with childhood friends Willow and Xander. Slightly different in that the "stranger" is the protagonist, not one of the friends.
 * The group grows and shrinks over the seasons, but these three remain the core. Creepy variant here is that Willow and Xander were actually already part of a Two Guys and a Girl group; Xander's best-friend-since-even-before-Willow, Jesse, gets turned into a vampire and staked in the premiere and is never mentioned again.
 * Buffy replaces him, except she's Willow's best friend instead of Xander's, and their lives now revolve around her world-saving gig. It's kind of like a less-extreme version of that season four Mary Sue spell Jonathon did.
 * Angel also starts out like this. Angel moves to LA and joins up with Cordelia, who he knew in Sunnydale for the three previous years, and "stranger" Doyle.
 * What I Like About You: Holly (main character), Gary (the friend), and Tina (the stranger).
 * Doctor Who (counting backwards):
 * Doctor (main), Amy (friend), Rory (stranger).
 * Amy (main), Rory (husband), Doctor (stranger).
 * Rory (main), Amy (wife), Doctor (stranger).
 * Doctor (main), Donna (friend), River (stranger).
 * Donna (main), Doctor (friend), River (stranger).
 * River (main), Doctor (friend), Donna (stranger).
 * Donna (main), Doctor (friend), Martha (stranger).
 * Martha (main), Doctor (friend), Donna (stranger).
 * Martha (main), Doctor (friend), Jack (stranger).
 * Jack (main), Doctor (friend), Martha (stranger).
 * Rose (main), Doctor (friend), Sarah-Jane (stranger).
 * Sarah-Jane (main), Doctor (friend), Rose (stranger).
 * Doctor (main), Rose (friend), Harriet (stranger).
 * Rose (main), Doctor (friend), Harriet (stranger).
 * Doctor (main), Rose ([wished to be more than a] friend), Jack (stranger).
 * Rose (main), Doctor ([wished to be more than a] friend), Jack (stranger).
 * Doctor (main), Rose (friend), Mickey (stranger).
 * Rose (main), Mickey ([ex-boy]friend), Doctor (stranger).
 * H₂O: Just Add Water has an all girl set-up for both main trios. Season 1-2 trio has Emma and Cleo as a set-up friendship, joined by new girl Rikki. Season 3 has the set-up restarted with Cleo and Rikki's friendship stabilized by two seasons, and Bella fills out the newcomer role.
 * Power Rangers Jungle Fury has Casey (Rookie Red Ranger), Theo, and Lily (friends).

Video Games

 * Chrono Trigger has hero Crono, his childhood friend Lucca, and stranger Marle.
 * Skies of Arcadia has childhood friends Vyse and Aika, joined by newcomer Mysterious Waif Fina.
 * Wild ARMs 5 has Dean and his friend Rebecca meet Mysterious Waif Avril.
 * Final Fantasy VII has this dynamic twice in your earliest parties. Childhood friends Cloud and Tifa are joined by stranger Barret and, later, Aeris(th).
 * Final Fantasy X has a variation where the hero is the stranger, with Yuna, Waka, and Lulu filling in for the friends.
 * In the sequel, Yuna and Rikku are the friends, with Paine as the stranger.
 * Disgaea has our "hero" Laharl and his childhood friend... er, vassal, Etna. When they are joined by Flonne the main plot kicks off.
 * Disgaea 3: Absence of Justice: "hero" Mao, "childhood friend" Rasberyl, and stranger Almaz. Of course, being a Disgaea game, the roles are played with and parodied at every opportunity.
 * Soul Nomad: Hero Revya, childhood friend Danette, and stranger Gig.
 * Legend of Dragoon switches this up with a guy from out of nowhere and the childhood friend being the Mysterious Waif. This holds until Rose joins the party, which results in the trope played fully straight with the previous guy from out of nowhere being an extra.
 * Golden Sun: The Lost Age has Felix, the hero; Jenna, his sister; and Sheba, the girl from nowhere. (Literally.)
 * A variation in Mass Effect, where Shepard and Kaidan had already been serving on the same ship for some time, run into Ashley very early in the game and bring her into the Normandy.
 * In Dragon Age II, your first party members upon reaching Kirkwall are your siblings and Aveline, the latter of whom you met while fleeing Lothering.
 * In World of Warcraft, you are the "stranger" to Fiona, Tarenar and Gidwin in their caravan quest chain in the new version of the Eastern Plaguelands. This dynamic happens quite often when guild groups queue in the Random Dungeon Finder and the empty slots are filled by randomly selected individuals.

Western Animation

 * Avatar: The Last Airbender: The main trio for Book 1 consists of Aang (stranger) alongside Katara and Sokka (sister and brother). As with Buffy, the stranger is the main character, though we're introduced to the siblings a few minutes before him.
 * Aang is the titular character, but Katara just barely edges out the rest of the True Companions for the protagonist slot for at least the first season. (By season two we have a split storyline with Zuko as beta protagonist and Aang steps into his 'lead' shoes a little more, as do the others. Katara still leads the pack, though, as original viewpoint character.)
 * The Legend of Korra follows suit - new Avatar Korra quickly teams up with pro-bending siblings Mako and Bolin.
 * Megas XLR, with all of its anime tropes being parodied, plays this one rather straight. It begins with Coop and his Hetero Life Mate Jamie, who are soon joined by Kiva, considerably more intelligent supersoldier future chick.